diff --git a/stage0/src/ast/attr.rs b/stage0/src/ast/attr.rs
index eeacb7c..6351003 100644
--- a/stage0/src/ast/attr.rs
+++ b/stage0/src/ast/attr.rs
@@ -48,6 +48,10 @@ impl Attributes {
         self.config.as_ref()
     }
 
+    pub fn ext(&self) -> Option<&(AttributeName, Extern)> {
+        self.ext.as_ref()
+    }
+
     pub fn repr(&self) -> Option<&(AttributeName, Representation)> {
         self.repr.as_ref()
     }
diff --git a/stage0/src/ast/func.rs b/stage0/src/ast/func.rs
index 883714a..59c37e8 100644
--- a/stage0/src/ast/func.rs
+++ b/stage0/src/ast/func.rs
@@ -1,12 +1,12 @@
-use super::{Attributes, Statement, Type};
-use crate::codegen::{Codegen, LlvmFunc, LlvmType, LlvmVoid};
-use crate::lexer::{FnKeyword, Identifier, SyntaxError};
+use super::{Attributes, Extern, Statement, Type};
+use crate::codegen::{BasicBlock, Builder, Codegen, LlvmFunc, LlvmType, LlvmVoid};
+use crate::lexer::{Identifier, SyntaxError};
+use std::borrow::Cow;
 use std::ffi::CString;
 
 /// A function.
 pub struct Function {
     attrs: Attributes,
-    def: FnKeyword,
     name: Identifier,
     params: Vec<FunctionParam>,
     ret: Option<Type>,
@@ -16,7 +16,6 @@ pub struct Function {
 impl Function {
     pub fn new(
         attrs: Attributes,
-        def: FnKeyword,
         name: Identifier,
         params: Vec<FunctionParam>,
         ret: Option<Type>,
@@ -24,7 +23,6 @@ impl Function {
     ) -> Self {
         Self {
             attrs,
-            def,
             name,
             params,
             ret,
@@ -44,8 +42,14 @@ impl Function {
             }
         }
 
+        // Build function name.
+        let name = match self.attrs.ext() {
+            Some((_, Extern::C)) => Cow::Borrowed(self.name.value()),
+            None => Cow::Owned(cx.encode_name(container, self.name.value())),
+        };
+
         // Check if function already exists.
-        let name = CString::new(cx.encode_name(container, self.name.value())).unwrap();
+        let name = CString::new(name.as_ref()).unwrap();
 
         if LlvmFunc::get(cx, &name).is_some() {
             return Err(SyntaxError::new(
@@ -85,11 +89,35 @@ impl Function {
         };
 
         // Create a function.
-        let func = LlvmFunc::new(cx, name, &params, ret);
+        let mut func = LlvmFunc::new(cx, name, &params, ret);
+
+        match &self.body {
+            Some(v) => Self::build_body(cx, &mut func, v),
+            None => {
+                if self.attrs.ext().is_none() {
+                    return Err(SyntaxError::new(
+                        self.name.span().clone(),
+                        "a body is required for non-extern or non-abstract",
+                    ));
+                }
+            }
+        }
 
-        // TODO: Build function body.
         Ok(Some(func))
     }
+
+    fn build_body<'a, 'b: 'a>(
+        cx: &'a Codegen<'b>,
+        func: &mut LlvmFunc<'a, 'b>,
+        stmts: &[Statement],
+    ) {
+        let mut bb = BasicBlock::new(cx);
+        let mut b = Builder::new(cx, &mut bb);
+
+        b.ret_void();
+
+        func.append(bb);
+    }
 }
 
 /// A function parameter.
diff --git a/stage0/src/ast/mod.rs b/stage0/src/ast/mod.rs
index 9be8749..30a93dc 100644
--- a/stage0/src/ast/mod.rs
+++ b/stage0/src/ast/mod.rs
@@ -10,8 +10,7 @@ pub use self::ty::*;
 pub use self::using::*;
 
 use crate::lexer::{
-    ClassKeyword, FnKeyword, Identifier, ImplKeyword, Lexer, StructKeyword, SyntaxError, Token,
-    UseKeyword,
+    ClassKeyword, Identifier, ImplKeyword, Lexer, StructKeyword, SyntaxError, Token, UseKeyword,
 };
 use std::path::PathBuf;
 use thiserror::Error;
@@ -342,8 +341,8 @@ impl SourceFile {
 
             match tok {
                 Token::AttributeName(name) => attrs = Some(Attributes::parse(lex, name)?),
-                Token::FnKeyword(def) => {
-                    functions.push(Self::parse_fn(lex, attrs.take().unwrap_or_default(), def)?);
+                Token::FnKeyword(_) => {
+                    functions.push(Self::parse_fn(lex, attrs.take().unwrap_or_default())?);
                 }
                 Token::CloseCurly(_) => break,
                 t => return Err(SyntaxError::new(t.span().clone(), "syntax error")),
@@ -353,11 +352,7 @@ impl SourceFile {
         Ok(TypeImpl::new(def, ty, functions))
     }
 
-    fn parse_fn(
-        lex: &mut Lexer,
-        attrs: Attributes,
-        def: FnKeyword,
-    ) -> Result<Function, SyntaxError> {
+    fn parse_fn(lex: &mut Lexer, attrs: Attributes) -> Result<Function, SyntaxError> {
         let name = lex.next_ident()?;
 
         // Parse parameters.
@@ -416,7 +411,7 @@ impl SourceFile {
         };
 
         let ret = match next {
-            Token::Semicolon(_) => return Ok(Function::new(attrs, def, name, params, None, None)),
+            Token::Semicolon(_) => return Ok(Function::new(attrs, name, params, None, None)),
             Token::OpenCurly(_) => None,
             Token::Colon(_) => {
                 let ret = Self::parse_type(lex)?;
@@ -432,7 +427,7 @@ impl SourceFile {
 
                 match next {
                     Token::Semicolon(_) => {
-                        return Ok(Function::new(attrs, def, name, params, Some(ret), None));
+                        return Ok(Function::new(attrs, name, params, Some(ret), None));
                     }
                     Token::OpenCurly(_) => {}
                     t => {
@@ -456,7 +451,7 @@ impl SourceFile {
         // Parse body.
         let body = Statement::parse_block(lex)?;
 
-        Ok(Function::new(attrs, def, name, params, ret, Some(body)))
+        Ok(Function::new(attrs, name, params, ret, Some(body)))
     }
 
     fn parse_type(lex: &mut Lexer) -> Result<Type, SyntaxError> {
diff --git a/stage0/src/codegen/block.rs b/stage0/src/codegen/block.rs
new file mode 100644
index 0000000..c53fe8b
--- /dev/null
+++ b/stage0/src/codegen/block.rs
@@ -0,0 +1,29 @@
+use super::Codegen;
+use llvm_sys::core::{LLVMCreateBasicBlockInContext, LLVMDeleteBasicBlock};
+use llvm_sys::prelude::LLVMBasicBlockRef;
+use std::marker::PhantomData;
+
+/// Encapsulate an LLVM basic block.
+pub struct BasicBlock<'a, 'b: 'a> {
+    value: LLVMBasicBlockRef,
+    phantom: PhantomData<&'a Codegen<'b>>,
+}
+
+impl<'a, 'b: 'a> BasicBlock<'a, 'b> {
+    pub fn new(cx: &'a Codegen<'b>) -> Self {
+        Self {
+            value: unsafe { LLVMCreateBasicBlockInContext(cx.llvm, b"\0".as_ptr() as _) },
+            phantom: PhantomData,
+        }
+    }
+
+    pub fn as_raw(&self) -> LLVMBasicBlockRef {
+        self.value
+    }
+}
+
+impl<'a, 'b: 'a> Drop for BasicBlock<'a, 'b> {
+    fn drop(&mut self) {
+        unsafe { LLVMDeleteBasicBlock(self.value) };
+    }
+}
diff --git a/stage0/src/codegen/builder.rs b/stage0/src/codegen/builder.rs
new file mode 100644
index 0000000..27bac27
--- /dev/null
+++ b/stage0/src/codegen/builder.rs
@@ -0,0 +1,35 @@
+use super::{BasicBlock, Codegen};
+use llvm_sys::core::{
+    LLVMBuildRetVoid, LLVMCreateBuilderInContext, LLVMDisposeBuilder, LLVMPositionBuilderAtEnd,
+};
+use llvm_sys::prelude::LLVMBuilderRef;
+use std::marker::PhantomData;
+
+/// Encapsulate an LLVM IR builder.
+pub struct Builder<'a, 'b: 'a> {
+    raw: LLVMBuilderRef,
+    phantom: PhantomData<&'a Codegen<'b>>,
+}
+
+impl<'a, 'b: 'a> Builder<'a, 'b> {
+    pub fn new(cx: &'a Codegen<'b>, block: &mut BasicBlock<'a, 'b>) -> Self {
+        let raw = unsafe { LLVMCreateBuilderInContext(cx.llvm) };
+
+        unsafe { LLVMPositionBuilderAtEnd(raw, block.as_raw()) };
+
+        Self {
+            raw,
+            phantom: PhantomData,
+        }
+    }
+
+    pub fn ret_void(&mut self) {
+        unsafe { LLVMBuildRetVoid(self.raw) };
+    }
+}
+
+impl<'a, 'b: 'a> Drop for Builder<'a, 'b> {
+    fn drop(&mut self) {
+        unsafe { LLVMDisposeBuilder(self.raw) };
+    }
+}
diff --git a/stage0/src/codegen/func.rs b/stage0/src/codegen/func.rs
index f574135..d02bdcd 100644
--- a/stage0/src/codegen/func.rs
+++ b/stage0/src/codegen/func.rs
@@ -1,8 +1,11 @@
-use super::{Codegen, LlvmType};
-use llvm_sys::core::{LLVMAddFunction, LLVMFunctionType, LLVMGetNamedFunction};
+use super::{BasicBlock, Codegen, LlvmType};
+use llvm_sys::core::{
+    LLVMAddFunction, LLVMAppendExistingBasicBlock, LLVMFunctionType, LLVMGetNamedFunction,
+};
 use llvm_sys::prelude::{LLVMTypeRef, LLVMValueRef};
 use std::ffi::CStr;
 use std::marker::PhantomData;
+use std::mem::forget;
 
 /// A function.
 pub struct LlvmFunc<'a, 'b: 'a> {
@@ -47,4 +50,9 @@ impl<'a, 'b: 'a> LlvmFunc<'a, 'b> {
             phantom: PhantomData,
         }
     }
+
+    pub fn append(&mut self, block: BasicBlock<'a, 'b>) {
+        unsafe { LLVMAppendExistingBasicBlock(self.value, block.as_raw()) };
+        forget(block);
+    }
 }
diff --git a/stage0/src/codegen/mod.rs b/stage0/src/codegen/mod.rs
index 981ba7a..0f1713a 100644
--- a/stage0/src/codegen/mod.rs
+++ b/stage0/src/codegen/mod.rs
@@ -1,3 +1,5 @@
+pub use self::block::*;
+pub use self::builder::*;
 pub use self::func::*;
 pub use self::resolver::*;
 pub use self::target::*;
@@ -22,6 +24,8 @@ use llvm_sys::target_machine::{
 use std::ffi::CStr;
 use std::ptr::{null, null_mut};
 
+mod block;
+mod builder;
 mod func;
 mod resolver;
 mod target;