diff --git a/src/asg/expr/kind.rs b/src/asg/expr/kind.rs
index 5efff1c..2573bf3 100644
--- a/src/asg/expr/kind.rs
+++ b/src/asg/expr/kind.rs
@@ -42,6 +42,7 @@ pub enum ExprKind {
     EnumMemberLiteral(Box<EnumMemberLiteral>),
     ResolvedNamedExpression(Box<Expr>),
     Zeroed(Box<Type>),
+    SizeOf(Box<Type>),
     InterpreterSyscall(InterpreterSyscallKind, Vec<Expr>),
 }
 
diff --git a/src/ast/expr/kind.rs b/src/ast/expr/kind.rs
index 0941c8b..911477f 100644
--- a/src/ast/expr/kind.rs
+++ b/src/ast/expr/kind.rs
@@ -3,7 +3,11 @@ use super::{
     InterpreterSyscall, ShortCircuitingBinaryOperation, StaticMemberCall, StaticMemberValue,
     StructLiteral, UnaryOperation, While,
 };
-use crate::{ast::Privacy, name::Name, source_files::Source};
+use crate::{
+    ast::{Privacy, Type},
+    name::Name,
+    source_files::Source,
+};
 use std::ffi::CString;
 
 #[derive(Clone, Debug)]
@@ -29,6 +33,7 @@ pub enum ExprKind {
     While(Box<While>),
     StaticMemberValue(Box<StaticMemberValue>),
     StaticMemberCall(Box<StaticMemberCall>),
+    SizeOf(Box<Type>),
     InterpreterSyscall(Box<InterpreterSyscall>),
 }
 
diff --git a/src/lower/expr/mod.rs b/src/lower/expr/mod.rs
index 312e937..e2403dc 100644
--- a/src/lower/expr/mod.rs
+++ b/src/lower/expr/mod.rs
@@ -137,6 +137,9 @@ pub fn lower_expr(
             Box::new(ir::Type::Void),
         )))),
         ExprKind::Call(call) => lower_expr_call(builder, ir_module, expr, function, asg, call),
+        ExprKind::PolyCall(poly_call) => {
+            lower_expr_poly_call(builder, ir_module, expr, function, asg, poly_call)
+        }
         ExprKind::Variable(variable) => {
             let pointer_to_variable = lower_variable_to_value(variable.key);
             let variable_type = lower_type(ir_module, &builder.unpoly(&variable.ty)?, asg)?;
@@ -484,6 +487,10 @@ pub fn lower_expr(
             let ir_type = lower_type(ir_module, &builder.unpoly(ty)?, asg)?;
             Ok(ir::Value::Literal(Literal::Zeroed(ir_type)))
         }
+        ExprKind::SizeOf(ty) => {
+            let ir_type = lower_type(ir_module, &builder.unpoly(ty)?, asg)?;
+            Ok(builder.push(ir::Instr::SizeOf(ir_type)))
+        }
         ExprKind::InterpreterSyscall(syscall, args) => {
             let mut values = Vec::with_capacity(args.len());
 
@@ -493,9 +500,6 @@ pub fn lower_expr(
 
             Ok(builder.push(ir::Instr::InterpreterSyscall(*syscall, values)))
         }
-        ExprKind::PolyCall(poly_call) => {
-            lower_expr_poly_call(builder, ir_module, expr, function, asg, poly_call)
-        }
     }
 }
 
diff --git a/src/parser/parse_expr/primary/mod.rs b/src/parser/parse_expr/primary/mod.rs
index ee73f66..d73ebd1 100644
--- a/src/parser/parse_expr/primary/mod.rs
+++ b/src/parser/parse_expr/primary/mod.rs
@@ -7,8 +7,8 @@ mod struct_literal;
 use super::{super::error::ParseError, is_right_associative, is_terminating_token, Parser};
 use crate::{
     ast::{
-        Block, Conditional, Expr, ExprKind, Integer, TypeKind, UnaryMathOperator, UnaryOperation,
-        UnaryOperator, While,
+        Block, Conditional, Expr, ExprKind, Integer, TypeArg, TypeKind, UnaryMathOperator,
+        UnaryOperation, UnaryOperator, While,
     },
     inflow::Inflow,
     parser::{array_last, error::ParseErrorKind, parse_util::into_plain_name},
@@ -148,9 +148,17 @@ impl<'a, I: Inflow<Token>> Parser<'a, I> {
                     }
                     _ => {
                         if !generics.is_empty() {
+                            // TODO: CLEANUP: Clean up this code
                             if let Some("sizeof") = name.as_plain_str() {
-                                todo!("sizeof operator");
-                                // Ok(Expr::new(ExprKind::SizeOf(inner), source))
+                                if let Some(type_arg) = generics.first() {
+                                    if let TypeArg::Type(ty) = type_arg {
+                                        if generics.len() == 1 {
+                                            return Ok(
+                                                ExprKind::SizeOf(Box::new(ty.clone())).at(source)
+                                            );
+                                        }
+                                    }
+                                }
                             }
 
                             return Err(ParseErrorKind::Other {
diff --git a/src/resolve/expr/mod.rs b/src/resolve/expr/mod.rs
index dcceae9..bab2d23 100644
--- a/src/resolve/expr/mod.rs
+++ b/src/resolve/expr/mod.rs
@@ -24,11 +24,15 @@ use super::{
     Initialized, ResolveTypeCtx,
 };
 use crate::{
-    asg::{self, Asg, CurrentConstraints, Expr, ExprKind, FuncRef, StructRef, TypeKind, TypedExpr},
+    asg::{
+        self, Asg, CurrentConstraints, Expr, ExprKind, FuncRef, IntegerBits, StructRef, TypeKind,
+        TypedExpr,
+    },
     ast::{
         self, CInteger, CIntegerAssumptions, ConformBehavior, IntegerKnown, Language, Settings,
         UnaryOperator,
     },
+    ir::IntegerSign,
     resolve::{
         error::ResolveErrorKind,
         expr::{
@@ -371,6 +375,14 @@ pub fn resolve_expr(
         ast::ExprKind::StaticMemberCall(static_access_call) => {
             resolve_static_member_call(ctx, static_access_call)
         }
+        ast::ExprKind::SizeOf(ast_type) => {
+            let ty = ctx.type_ctx().resolve(ast_type)?;
+
+            Ok(TypedExpr::new(
+                asg::TypeKind::Integer(IntegerBits::Bits64, IntegerSign::Unsigned).at(source),
+                asg::ExprKind::SizeOf(Box::new(ty)).at(source),
+            ))
+        }
         ast::ExprKind::InterpreterSyscall(info) => {
             let ast::InterpreterSyscall {
                 kind,