diff --git a/src/frontend.rs b/src/frontend.rs
index 999a6b7..94442e8 100644
--- a/src/frontend.rs
+++ b/src/frontend.rs
@@ -110,29 +110,62 @@ struct FunctionBodyBuilder<'a, 'b> {
 #[derive(Debug)]
 enum Frame {
     Block {
+        start_depth: usize,
         out: BlockId,
         params: Vec<Type>,
         results: Vec<Type>,
     },
     Loop {
-        top: BlockId,
+        start_depth: usize,
+        header: BlockId,
         out: BlockId,
         params: Vec<Type>,
         results: Vec<Type>,
     },
     If {
+        start_depth: usize,
         out: BlockId,
         el: BlockId,
+        param_values: Vec<ValueId>,
         params: Vec<Type>,
         results: Vec<Type>,
     },
     Else {
+        start_depth: usize,
         out: BlockId,
         params: Vec<Type>,
         results: Vec<Type>,
     },
 }
 
+impl Frame {
+    fn start_depth(&self) -> usize {
+        match self {
+            Frame::Block { start_depth, .. }
+            | Frame::Loop { start_depth, .. }
+            | Frame::If { start_depth, .. }
+            | Frame::Else { start_depth, .. } => *start_depth,
+        }
+    }
+
+    fn br_args(&self) -> &[Type] {
+        match self {
+            Frame::Block { results, .. }
+            | Frame::If { results, .. }
+            | Frame::Else { results, .. } => &results[..],
+            Frame::Loop { params, .. } => &params[..],
+        }
+    }
+
+    fn br_target(&self) -> BlockId {
+        match self {
+            Frame::Block { out, .. } => *out,
+            Frame::Loop { header, .. } => *header,
+            Frame::If { out, .. } | Frame::Else { out, .. } => *out,
+        }
+    }
+}
+
 impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
     fn new(module: &'b Module<'a>, my_sig: SignatureId, body: &'b mut FunctionBody<'a>) -> Self {
         body.blocks.push(Block::default());
@@ -146,8 +179,12 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         }
     }
 
+    fn add_block_params(&mut self, block: BlockId, tys: &[Type]) {
+        self.body.blocks[block].params.extend_from_slice(tys);
+    }
+
     fn handle_op(&mut self, op: Operator<'a>) -> Result<()> {
-        match op {
+        match &op {
             Operator::Unreachable
             | Operator::Call { .. }
             | Operator::LocalGet { .. }
@@ -158,27 +195,171 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
                 None => {
                     self.emit(Operator::Return)?;
                 }
-                Some(Frame::Block { out, .. }) | Some(Frame::Loop { out, .. }) => {
-                    // No need to manipulate stack: assuming the input
-                    // Wasm was validated properly, the `results`
-                    // values must be on the top of the stack now, and
-                    // they will remain so once we exit the block.
-                    self.emit_branch(out);
+                Some(Frame::Block {
+                    start_depth,
+                    out,
+                    results,
+                    ..
+                })
+                | Some(Frame::Loop {
+                    start_depth,
+                    out,
+                    results,
+                    ..
+                }) => {
+                    // Generate a branch to the out-block with
+                    // blockparams for the results.
+                    let result_values = self.op_stack.split_off(results.len());
+                    self.emit_branch(out, &result_values[..]);
+                    assert_eq!(self.op_stack.len(), start_depth);
                     self.cur_block = out;
+                    self.push_block_params(&results[..]);
+                }
+                Some(Frame::If {
+                    start_depth,
+                    out,
+                    el,
+                    param_values,
+                    results,
+                    ..
+                }) => {
+                    // Generate a branch to the out-block with
+                    // blockparams for the results.
+                    let result_values = self.op_stack.split_off(results.len());
+                    self.emit_branch(out, &result_values[..]);
+                    // No `else`, so we need to generate a trivial
+                    // branch in the else-block. If the if-block-type
+                    // has results, they must be exactly the params.
+                    let else_result_values = param_values;
+                    assert_eq!(else_result_values.len(), results.len());
+                    self.emit_branch(el, &else_result_values[..]);
+                    assert_eq!(self.op_stack.len(), start_depth);
+                    self.cur_block = out;
+                    self.push_block_params(&results[..]);
+                }
+                Some(Frame::Else {
+                    out,
+                    results,
+                    start_depth,
+                    ..
+                }) => {
+                    // Generate a branch to the out-block with
+                    // blockparams for the results.
+                    let result_values = self.op_stack.split_off(results.len());
+                    assert_eq!(self.op_stack.len(), start_depth);
+                    self.emit_branch(out, &result_values[..]);
+                    self.cur_block = out;
+                    self.push_block_params(&results[..]);
                 }
-                _ => bail!("unsupported block type"),
             },
 
             Operator::Block { ty } => {
-                let (params, results) = self.block_params_and_results(ty);
+                let (params, results) = self.block_params_and_results(*ty);
                 let out = self.create_block();
+                self.add_block_params(out, &results[..]);
+                let start_depth = self.op_stack.len() - params.len();
                 self.ctrl_stack.push(Frame::Block {
+                    start_depth,
                     out,
                     params,
                     results,
                 });
             }
 
+            Operator::Loop { ty } => {
+                let (params, results) = self.block_params_and_results(*ty);
+                let header = self.create_block();
+                self.add_block_params(header, &params[..]);
+                let initial_args = self.op_stack.split_off(params.len());
+                let start_depth = self.op_stack.len();
+                self.emit_branch(header, &initial_args[..]);
+                self.cur_block = header;
+                self.push_block_params(&params[..]);
+                let out = self.create_block();
+                self.ctrl_stack.push(Frame::Loop {
+                    start_depth,
+                    header,
+                    out,
+                    params,
+                    results,
+                });
+            }
+
+            Operator::If { ty } => {
+                let (params, results) = self.block_params_and_results(*ty);
+                let if_true = self.create_block();
+                let if_false = self.create_block();
+                let join = self.create_block();
+                self.add_block_params(join, &results[..]);
+                let cond = self.op_stack.pop().unwrap();
+                let param_values = self.op_stack[self.op_stack.len() - params.len()..].to_vec();
+                let start_depth = self.op_stack.len();
+                self.ctrl_stack.push(Frame::If {
+                    start_depth,
+                    out: join,
+                    el: if_false,
+                    param_values,
+                    params,
+                    results,
+                });
+                self.cur_block = if_true;
+                self.emit_cond_branch(cond, if_true, &[], if_false, &[]);
+            }
+
+            Operator::Else => {
+                if let Frame::If {
+                    start_depth,
+                    out,
+                    el,
+                    param_values,
+                    params,
+                    results,
+                } = self.ctrl_stack.pop().unwrap()
+                {
+                    let if_results = self.op_stack.split_off(results.len());
+                    self.emit_branch(out, &if_results[..]);
+                    self.op_stack.extend(param_values);
+                    self.ctrl_stack.push(Frame::Else {
+                        start_depth,
+                        out,
+                        params,
+                        results,
+                    });
+                    self.cur_block = el;
+                } else {
+                    bail!("Else without If on top of frame stack");
+                }
+            }
+
+            Operator::Br { relative_depth } | Operator::BrIf { relative_depth } => {
+                let cond = match &op {
+                    Operator::Br { .. } => None,
+                    Operator::BrIf { .. } => Some(self.op_stack.pop().unwrap()),
+                    _ => unreachable!(),
+                };
+                // Pop skipped-over frames.
+                let _ = self.ctrl_stack.split_off(*relative_depth as usize);
+                // Get the frame we're branching to.
+                let frame = self.ctrl_stack.pop().unwrap();
+                // Get the args off the stack.
+                let args = self.op_stack.split_off(frame.br_args().len());
+                // Truncate the result stack down to the expected height.
+                self.op_stack.truncate(frame.start_depth());
+                // Finally, generate the branch itself.
+                match cond {
+                    None => {
+                        self.emit_branch(frame.br_target(), &args[..]);
+                    }
+                    Some(cond) => {
+                        let cont = self.create_block();
+                        self.emit_cond_branch(cond, frame.br_target(), &args[..], cont, &[]);
+                        self.cur_block = cont;
+                    }
+                }
+            }
+
+            Operator::BrTable { .. } => {}
+
             _ => bail!("Unsupported operator: {:?}", op),
         }
 
@@ -204,11 +385,58 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         }
     }
 
-    fn emit_branch(&mut self, target: BlockId) {
+    fn emit_branch(&mut self, target: BlockId, args: &[ValueId]) {
         let block = self.cur_block;
+        let args = args.iter().map(|&val| Operand::Value(val)).collect();
+        let target = BlockTarget {
+            block: target,
+            args,
+        };
         self.body.blocks[block].terminator = Terminator::Br { target };
     }
 
+    fn emit_cond_branch(
+        &mut self,
+        cond: ValueId,
+        if_true: BlockId,
+        if_true_args: &[ValueId],
+        if_false: BlockId,
+        if_false_args: &[ValueId],
+    ) {
+        let block = self.cur_block;
+        let if_true_args = if_true_args
+            .iter()
+            .map(|&val| Operand::Value(val))
+            .collect();
+        let if_false_args = if_false_args
+            .iter()
+            .map(|&val| Operand::Value(val))
+            .collect();
+        self.body.blocks[block].terminator = Terminator::CondBr {
+            cond: Operand::Value(cond),
+            if_true: BlockTarget {
+                block: if_true,
+                args: if_true_args,
+            },
+            if_false: BlockTarget {
+                block: if_false,
+                args: if_false_args,
+            },
+        };
+    }
+
+    fn push_block_params(&mut self, tys: &[Type]) {
+        assert_eq!(tys, self.body.blocks[self.cur_block].params);
+        for (i, &ty) in tys.iter().enumerate() {
+            let value_id = self.body.values.len() as ValueId;
+            self.body.values.push(ValueDef {
+                kind: ValueKind::BlockParam(self.cur_block, i),
+                ty,
+            });
+            self.op_stack.push(value_id);
+        }
+    }
+
     fn emit(&mut self, op: Operator<'a>) -> Result<()> {
         let block = self.cur_block;
         let inst = self.body.blocks[block].insts.len() as InstId;
diff --git a/src/ir.rs b/src/ir.rs
index f783fd4..eac9af7 100644
--- a/src/ir.rs
+++ b/src/ir.rs
@@ -68,20 +68,26 @@ pub enum Operand<'a> {
     Sub(Box<Inst<'a>>),
 }
 
+#[derive(Clone, Debug)]
+pub struct BlockTarget<'a> {
+    pub block: BlockId,
+    pub args: Vec<Operand<'a>>,
+}
+
 #[derive(Clone, Debug)]
 pub enum Terminator<'a> {
     Br {
-        target: BlockId,
+        target: BlockTarget<'a>,
     },
     CondBr {
         cond: Operand<'a>,
-        if_true: BlockId,
-        if_false: BlockId,
+        if_true: BlockTarget<'a>,
+        if_false: BlockTarget<'a>,
     },
     Select {
         value: Operand<'a>,
-        targets: Vec<BlockId>,
-        default: BlockId,
+        targets: Vec<BlockTarget<'a>>,
+        default: BlockTarget<'a>,
     },
     Return {
         values: Vec<Operand<'a>>,