diff --git a/crates/mist-api/src/transpiler.mist b/crates/mist-api/src/transpiler.mist
index f2975fe..f7001ea 100644
--- a/crates/mist-api/src/transpiler.mist
+++ b/crates/mist-api/src/transpiler.mist
@@ -184,8 +184,13 @@ fn is_source_newer(source *Path, output *Path) std::io::Result<bool> {
     Ok(source_time > output_time)
 }
 
+pub enum TranspileError<'a> {
+    Parse[error::ParseError<'a, Vec<mist_parser::ast::TopLevel>>],
+    Semantic[Vec<mist_parser::semantics::SemanticError>]
+}
+
 pub fn transpile_code<'a>(path *PathBuf, source *'a str, mut output_file PathBuf, mod_decl *str, curr_mod_decl *Option<[mist_parser::ast::Visibility, mist_parser::ast::Identifier]>)
-        Result<[PathBuf, mist_codegen::RustCodegen, String], error::ParseError<'a, Vec<mist_parser::ast::TopLevel>>> {
+        Result<[PathBuf, mist_codegen::RustCodegen, String], TranspileError<'a>> {
 
     if let Some[[_, name]] = &curr_mod_decl {
         if output_file.file_name().map(fn(v) -> v.to_str()).unwrap_or_default() != Some("mod.rs") {
@@ -204,7 +209,11 @@ pub fn transpile_code<'a>(path *PathBuf, source *'a str, mut output_file PathBuf
 
     let mut gc = mist_codegen::RustCodegen::new(path.clone());
 
-    let parsed = mist_parser::parse(source)?;
+    let parsed = mist_parser::parse(source).map_err(TranspileError::Parse)?;
+
+    for i in &parsed {
+        let _ = mist_parser::semantics::check_class_semantics(i).map_err(TranspileError::Semantic)?;
+    }
 
     let output = gc.generate(parsed);
 
@@ -217,32 +226,8 @@ pub fn transpile_file(path *PathBuf, source *str, output_file PathBuf, mod_decl
     let [output_file, gc, output] = match transpile_code(path, source, output_file, mod_decl, curr_mod_decl) {
         Ok[output] => output;
         Err[e] => {
-            match e {
-                error::ParseError::Ast[e] => {
-                    let start_pos = e.span.start_pos().line_col();
-
-                    let span = e.span.as_str();
-
-                    eprintln!(
-                        "\n{}:{}:{}\n \x1b[31mError\x1b[0m: {}\n\t{}{}\t{}",
-                        path.as_os_str().display(),
-                        start_pos.0,
-                        start_pos.1,
-                        e.error_message,
-                        span,
-                        if span.ends_with("\n") { "" } else { "\n" },
-                        "^".repeat(span.trim().len()),
-                    );
-
-                    process::exit(1);
-                }
-
-                error::ParseError::PreAst[e] => {
-                    eprintln!("error: parse failed in {}\n{}", path.display(), e);
-
-                    process::exit(1);
-                }
-            }
+            decode_err(&e, path);
+            process::exit(1);
         }
     };
 
@@ -272,3 +257,46 @@ pub fn transpile_file(path *PathBuf, source *str, output_file PathBuf, mod_decl
         process::exit(1);
     }
 }
+
+pub fn decode_err<'a>(err *TranspileError<'a>, path *PathBuf) {
+    let curr_dir = std::env::current_dir().unwrap();
+
+    match err {
+        TranspileError::Semantic[e_list] => {
+            for e in e_list {
+                eprintln!(
+                    "\n{}:{}:{}\n \x1b[31mError\x1b[0m: {}",
+                    path.strip_prefix(&curr_dir).unwrap_or_else(|_| path).as_os_str().display(),
+                    e.line,
+                    e.column,
+                    e.error_message,
+                );
+            }
+        }
+
+        TranspileError::Parse[e] => {
+            match e {
+                error::ParseError::Ast[e] => {
+                    let start_pos = e.span.start_pos().line_col();
+
+                    let span = e.span.as_str();
+
+                    eprintln!(
+                        "\n{}:{}:{}\n \x1b[31mError\x1b[0m: {}\n\t{}{}\t{}",
+                        path.strip_prefix(curr_dir).unwrap_or_else(|_| path).as_os_str().display(),
+                        start_pos.0,
+                        start_pos.1,
+                        e.error_message,
+                        span,
+                        if span.ends_with("\n") { "" } else { "\n" },
+                        "^".repeat(span.trim().len()),
+                    );
+                }
+
+                error::ParseError::PreAst[e] => {
+                    eprintln!("error: parse failed in {}\n{}", path.strip_prefix(curr_dir).unwrap_or_else(|_| path).display(), e);
+                }
+            }
+        }
+    }
+}
diff --git a/crates/parser/src/grammar.pest b/crates/parser/src/grammar.pest
index 5af7c30..561c979 100644
--- a/crates/parser/src/grammar.pest
+++ b/crates/parser/src/grammar.pest
@@ -333,7 +333,7 @@ trait_decl = {
 override_kw = { "override" ~ ("(" ~ expr_path ~ ")")? }
 
 function_decl = {
-    visibility? ~ override_kw? ~ "fn" ~ ("void" | type_expr) ~ identifier ~ generics_decl? ~ "(" ~ method_args ~ ")" ~ (block | semicolon)
+    visibility? ~ "fn" ~ ("void" | type_expr) ~ identifier ~ generics_decl? ~ "(" ~ method_args ~ ")" ~ override_kw? ~ (block | semicolon)
 }
 
 // ======================================================
diff --git a/crates/parser/src/lib.rs b/crates/parser/src/lib.rs
index ea39710..cf905f0 100644
--- a/crates/parser/src/lib.rs
+++ b/crates/parser/src/lib.rs
@@ -5,6 +5,7 @@ pub mod ast;
 pub mod error;
 pub mod parser;
 pub mod rev_mapper;
+pub mod semantics;
 
 use ast::*;
 
diff --git a/crates/parser/src/parser/items/function.rs b/crates/parser/src/parser/items/function.rs
index ef37078..10b519e 100644
--- a/crates/parser/src/parser/items/function.rs
+++ b/crates/parser/src/parser/items/function.rs
@@ -13,7 +13,6 @@ impl<'a> TryFrom<pest::iterators::Pair<'a, Rule>> for FunctionDecl {
         ast_ensure!(pair, Rule::function_decl => {
         let mut inner = pair.into_inner();
             let visibility = Visibility::try_from(&mut inner);
-            let is_override = consume_rule(&mut inner, Rule::override_kw).map(Override::try_from).transpose();
 
             let return_type = consume_rule(&mut inner, Rule::type_expr)
                 .map(TypeExpr::try_from)
@@ -64,6 +63,8 @@ impl<'a> TryFrom<pest::iterators::Pair<'a, Rule>> for FunctionDecl {
                 })
                 .unwrap_or_else(|| Ok(ParamList(self_param.into_iter().collect())));
 
+            let is_override = consume_rule(&mut inner, Rule::override_kw).map(Override::try_from).transpose();
+
             let body = inner.next().map(Block::try_from).transpose();
 
             ast_expr!(Self {
diff --git a/crates/parser/src/semantics.rs b/crates/parser/src/semantics.rs
new file mode 100644
index 0000000..0b1a9aa
--- /dev/null
+++ b/crates/parser/src/semantics.rs
@@ -0,0 +1,367 @@
+use crate::ast::{
+    Block, ClassItem, Expression, FunctionDecl, Identifier, MatchItem, Postfix, Prefix, Spanned,
+    Statement, StatementBranch, TopLevel, TopLevelKind, VarDeclStmt,
+};
+
+pub trait GetMutability {
+    fn get_mutability(&self) -> Vec<Identifier>;
+}
+
+#[derive(Debug, Clone)]
+pub struct SemanticError {
+    pub line: usize,
+    pub column: usize,
+    pub error_message: String,
+}
+
+/// Class semantics - Ensuring fields are initialized
+
+pub fn find_function_call<'a>(
+    items: &'a Vec<ClassItem>,
+    name: &Identifier,
+) -> Option<&'a Spanned<FunctionDecl>> {
+    items.iter().find_map(|v| {
+        if let ClassItem::Method(m) = v {
+            if m.item.is_using_self() && &m.item.name == name {
+                Some(m)
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    })
+}
+
+pub fn check_class_semantics<'a>(top_level: &TopLevel) -> Result<(), Vec<SemanticError>> {
+    if let TopLevelKind::ClassDecl {
+        fields,
+        constructor,
+        items,
+        name,
+        inherits,
+        ..
+    } = &top_level.0.item
+    {
+        if let Some(constructor) = constructor {
+            let mut fields = fields
+                .iter()
+                .map(|field| Spanned {
+                    item: field.item.decl.name.clone(),
+                    line: field.line,
+                    column: field.column,
+                })
+                .collect::<Vec<_>>();
+
+            if inherits.is_some() {
+                fields.push(Spanned {
+                    line: top_level.0.line,
+                    column: top_level.0.column,
+                    item: Identifier("_super".to_string()),
+                });
+            }
+
+            let mut mutability = constructor.item.body.get_mutability();
+
+            let mut i = 0;
+            while i < mutability.len() {
+                let name = mutability[i].clone();
+                if let Some(method) = find_function_call(items, &name) {
+                    if let Some(body) = &method.item.body {
+                        for entry in body.get_mutability() {
+                            if !mutability.contains(&entry) {
+                                mutability.push(entry);
+                            }
+                        }
+                    }
+                }
+                i += 1;
+            }
+
+            let mut errs = Vec::new();
+
+            for field in &fields {
+                if !mutability.contains(&field.item) {
+                    errs.push(SemanticError {
+                        line: field.line,
+                        column: field.column,
+                        error_message: format!(
+                            "class field `{}.{}` is uninitialized",
+                            name.0, field.item.0
+                        ),
+                    });
+                }
+            }
+
+            if errs.len() > 0 {
+                return Err(errs);
+            }
+        }
+
+        Ok(())
+    } else {
+        Ok(())
+    }
+}
+
+impl<T: GetMutability> GetMutability for Vec<T> {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        self.iter().flat_map(T::get_mutability).collect()
+    }
+}
+
+impl GetMutability for Block {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        let mut items: Vec<Identifier> = self
+            .statements
+            .iter()
+            .flat_map(|v| v.item.get_mutability())
+            .collect();
+
+        if let Some(v) = &self.soft_return {
+            items.append(&mut v.item.get_mutability());
+        }
+
+        items
+    }
+}
+
+impl GetMutability for Statement {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        match self {
+            Statement::Block(body) | Statement::UnsafeBlock(body) => body.get_mutability(),
+            Statement::If {
+                initial,
+                else_if,
+                else_branch,
+            } => {
+                let mut all_sets: Vec<Vec<Identifier>> = Vec::new();
+                all_sets.push(initial.body.get_mutability());
+
+                for branch in else_if {
+                    all_sets.push(branch.body.get_mutability());
+                }
+
+                if let Some(else_block) = else_branch {
+                    all_sets.push(else_block.get_mutability());
+                } else {
+                    return Vec::new();
+                }
+
+                let mut result = all_sets.remove(0);
+                for set in all_sets {
+                    result.retain(|id| set.contains(id));
+                }
+                result
+            }
+            Statement::Loop(body) => body.get_mutability(),
+            Statement::While(branch) => branch.body.get_mutability(),
+            Statement::CStyleFor {
+                body,
+                init,
+                condition,
+                update,
+                ..
+            } => {
+                let mut a = init.get_mutability();
+                a.append(&mut condition.get_mutability());
+                a.append(&mut update.get_mutability());
+                a.append(&mut body.get_mutability());
+                a
+            }
+            Statement::For { body, iterator, .. } => {
+                let mut a = iterator.get_mutability();
+                a.append(&mut body.get_mutability());
+                a
+            }
+            Statement::Match(_, items) => {
+                if items.is_empty() {
+                    return Vec::new();
+                }
+                let mut all_sets: Vec<Vec<Identifier>> = items
+                    .iter()
+                    .map(|item| item.item.get_mutability())
+                    .collect();
+                let mut result = all_sets.remove(0);
+                for set in all_sets {
+                    result.retain(|id| set.contains(id));
+                }
+                result
+            }
+            Statement::VarDecl(stmt) => stmt
+                .init
+                .as_ref()
+                .map(|e| e.get_mutability())
+                .unwrap_or_default(),
+            Statement::Return(expr) => expr
+                .as_ref()
+                .map(|e| e.get_mutability())
+                .unwrap_or_default(),
+            Statement::Break | Statement::Continue => Vec::new(),
+        }
+    }
+}
+
+impl GetMutability for StatementBranch {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        let mut a = self.condition.get_mutability();
+        a.append(&mut self.body.get_mutability());
+        a
+    }
+}
+
+impl GetMutability for MatchItem {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        self.1.get_mutability()
+    }
+}
+
+impl GetMutability for VarDeclStmt {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        self.init
+            .as_ref()
+            .map(|e| e.get_mutability())
+            .unwrap_or_default()
+    }
+}
+
+fn get_self_ref(initial: &Expression, postfixes: &[Postfix]) -> Option<Identifier> {
+    match initial {
+        Expression::Path(v) => {
+            let ident = v.0.first()?.ident.0.as_str();
+            if ident == "self" || ident == "super" {
+                if let Some(Postfix::FieldAccess(field, _)) = postfixes.first() {
+                    if !matches!(postfixes.get(1), Some(Postfix::Call(_))) {
+                        return Some(field.clone());
+                    }
+                }
+            }
+        }
+        _ => {}
+    }
+    None
+}
+
+fn get_self_method_call(initial: &Expression, postfixes: &[Postfix]) -> Option<Identifier> {
+    match initial {
+        Expression::Path(v) => {
+            let ident = v.0.first()?.ident.0.as_str();
+            if ident == "self" || ident == "super" {
+                if let Some(Postfix::FieldAccess(method, _)) = postfixes.first() {
+                    if matches!(postfixes.get(1), Some(Postfix::Call(_))) {
+                        return Some(method.clone());
+                    }
+                }
+            }
+        }
+        _ => {}
+    }
+    None
+}
+
+impl Expression {
+    fn get_self_ref(&self) -> Option<Identifier> {
+        match self {
+            Self::Fix {
+                initial, postfixes, ..
+            } => get_self_ref(initial, postfixes),
+            _ => None,
+        }
+    }
+}
+
+impl GetMutability for Expression {
+    fn get_mutability(&self) -> Vec<Identifier> {
+        match self {
+            Self::Array(v) => v.get_mutability(),
+            Self::ArrayRepeat(v, i) => {
+                let mut a = v.get_mutability();
+                a.append(&mut i.get_mutability());
+                a
+            }
+            Self::Binary { lhs, op, rhs } => {
+                if mutates_lhs(op) {
+                    let mut a = if let Some(self_ref) = lhs.get_self_ref() {
+                        vec![self_ref]
+                    } else {
+                        lhs.get_mutability()
+                    };
+
+                    a.append(&mut rhs.get_mutability());
+                    a
+                } else {
+                    let mut a = lhs.get_mutability();
+                    a.append(&mut rhs.get_mutability());
+                    a
+                }
+            }
+            Self::Statement(stmt) => stmt.get_mutability(),
+            Self::Fix {
+                initial,
+                prefixes,
+                postfixes,
+            } => {
+                let mut result = Vec::new();
+
+                // &mut self.field / &mut super.field / &mut super pattern
+                if prefixes.iter().any(|p| matches!(p, Prefix::RefMut)) {
+                    if let Some(self_ref) = get_self_ref(initial, postfixes) {
+                        result.push(self_ref);
+                    } else if let Expression::Path(v) = &**initial {
+                        if v.0.first().map_or(false, |s| s.ident.0 == "super") {
+                            result.push(Identifier("_super".to_string()));
+                        }
+                    }
+                }
+
+                // Recurse into all postfix sub-expressions
+                for postfix in postfixes {
+                    match postfix {
+                        Postfix::Assign(_, rhs) => {
+                            if let Some(self_ref) = get_self_ref(initial, postfixes) {
+                                result.push(self_ref);
+                            }
+                            result.append(&mut rhs.get_mutability());
+                        }
+                        Postfix::Call(args) => {
+                            for arg in args {
+                                result.append(&mut arg.get_mutability());
+                            }
+                        }
+                        Postfix::StructCall(fields) => {
+                            for (_, expr) in fields {
+                                if let Some(expr) = expr {
+                                    result.append(&mut expr.get_mutability());
+                                }
+                            }
+                        }
+                        Postfix::Index(expr) => {
+                            result.append(&mut expr.get_mutability());
+                        }
+                        _ => {}
+                    }
+                }
+
+                // Method calls on self: self.method_name(args)
+                if let Some(method) = get_self_method_call(initial, postfixes) {
+                    result.push(method);
+                }
+
+                result
+            }
+            Self::Closure { body, .. } => body.get_mutability(),
+            Self::Literal(_) => Vec::new(),
+            Self::Path(v) => {
+                if v.0.first().map_or(false, |s| s.ident.0 == "super") {
+                    vec![Identifier("_super".to_string())]
+                } else {
+                    Vec::new()
+                }
+            }
+        }
+    }
+}
+
+pub fn mutates_lhs(op: &str) -> bool {
+    matches!(op, "=" | "->")
+}
diff --git a/mist-test/src/items/class_test.mist b/mist-test/src/items/class_test.mist
index a95c15a..3d7ab57 100644
--- a/mist-test/src/items/class_test.mist
+++ b/mist-test/src/items/class_test.mist
@@ -1,8 +1,7 @@
 pub module class_test;
 
-pub class Animal<T> {
+pub class Animal {
     pub name String;
-    pub val T;
 
     constructor() {
         self.name = "Rex".to_string();
@@ -19,7 +18,7 @@ pub class Animal<T> {
     pub fn void static_method() {}
 }
 
-pub class Mammal<T> : Animal<T> {
+pub class Mammal : Animal {
     constructor() {
         super = Super::new();
     }
@@ -30,12 +29,12 @@ pub class Mammal<T> : Animal<T> {
     }
 }
 
-pub class Dog : Mammal<i32> {
+pub class Dog : Mammal {
     constructor() {
         super = Super::new();
     }
 
-    pub override(Animal<i32>) fn String speak(*self) {
+    pub fn String speak(*self) override(Animal) {
         "Woof!".to_string()
     }