diff --git a/compiler/gen/src/crane/convert.rs b/compiler/gen/src/crane/convert.rs
index 857cbdf8f9..f575052d62 100644
--- a/compiler/gen/src/crane/convert.rs
+++ b/compiler/gen/src/crane/convert.rs
@@ -10,7 +10,7 @@ pub fn type_from_layout(cfg: TargetFrontendConfig, layout: &Layout<'_>) -> Type
     use roc_mono::layout::Layout::*;
 
     match layout {
-        Pointer(_) | FunctionPointer(_, _) | Struct(_) | Tag(_) => cfg.pointer_type(),
+        Pointer(_) | FunctionPointer(_, _) | Struct(_) | Union(_) => cfg.pointer_type(),
         Builtin(builtin) => match builtin {
             Int64 => types::I64,
             Float64 => types::F64,
diff --git a/compiler/gen/src/llvm/convert.rs b/compiler/gen/src/llvm/convert.rs
index 2c8c16dd6e..c09068a420 100644
--- a/compiler/gen/src/llvm/convert.rs
+++ b/compiler/gen/src/llvm/convert.rs
@@ -68,7 +68,7 @@ pub fn basic_type_from_layout<'ctx>(
                 .struct_type(field_types.into_bump_slice(), false)
                 .as_basic_type_enum()
         }
-        Tag(_fields) => {
+        Union(_fields) => {
             panic!("TODO layout_to_basic_type for Tag");
         }
         Pointer(_layout) => {
diff --git a/compiler/mono/src/decision_tree.rs b/compiler/mono/src/decision_tree.rs
index 1c09630641..71d725a1b3 100644
--- a/compiler/mono/src/decision_tree.rs
+++ b/compiler/mono/src/decision_tree.rs
@@ -124,7 +124,9 @@ fn flatten_patterns<'a>(branch: Branch<'a>) -> Branch<'a> {
 
 fn flatten<'a>(path_pattern: (Path, Pattern<'a>), path_patterns: &mut Vec<(Path, Pattern<'a>)>) {
     match &path_pattern.1 {
-        Pattern::AppliedTag { union, .. } => {
+        Pattern::AppliedTag {
+            union, arguments, ..
+        } => {
             if union.alternatives.len() == 1 {
                 //        case map dearg ctorArgs of
                 //          [arg] ->
diff --git a/compiler/mono/src/expr.rs b/compiler/mono/src/expr.rs
index bdb16952bb..91cf15d6cb 100644
--- a/compiler/mono/src/expr.rs
+++ b/compiler/mono/src/expr.rs
@@ -104,6 +104,7 @@ pub struct Env<'a, 'i> {
     pub ident_ids: &'i mut IdentIds,
     pub pointer_size: u32,
     symbol_counter: usize,
+    pub jump_counter: &'a mut u64,
 }
 
 impl<'a, 'i> Env<'a, 'i> {
@@ -217,6 +218,7 @@ impl<'a> Expr<'a> {
             ident_ids,
             pointer_size,
             symbol_counter: 0,
+            jump_counter: arena.alloc(0),
         };
 
         from_can(&mut env, can_expr, procs, None)
diff --git a/compiler/mono/src/layout.rs b/compiler/mono/src/layout.rs
index 2a7c716e32..ea17bff3fa 100644
--- a/compiler/mono/src/layout.rs
+++ b/compiler/mono/src/layout.rs
@@ -10,7 +10,7 @@ use roc_types::subs::{Content, FlatType, Subs, Variable};
 pub enum Layout<'a> {
     Builtin(Builtin<'a>),
     Struct(&'a [(Lowercase, Layout<'a>)]),
-    Tag(&'a [Layout<'a>]),
+    Union(&'a MutMap<TagName, &'a [Layout<'a>]>),
     Pointer(&'a Layout<'a>),
     /// A function. The types of its arguments, then the type of its return value.
     FunctionPointer(&'a [Layout<'a>], &'a Layout<'a>),
@@ -93,12 +93,15 @@ impl<'a> Layout<'a> {
 
                 sum
             }
-            Tag(fields) => {
-                // the symbol is a 64-bit value, so 8 bytes
-                let mut sum = 8;
+            Union(fields) => {
+                // the tag gets converted to a u8, so 1 byte.
+                // But for one-tag unions, we don't store the tag, so 0 bytes
+                let mut sum = (fields.len() > 1) as u32;
 
-                for field_layout in *fields {
-                    sum += field_layout.stack_size(pointer_size);
+                for (_, tag_layout) in *fields {
+                    for field_layout in *tag_layout {
+                        sum += field_layout.stack_size(pointer_size);
+                    }
                 }
 
                 sum
@@ -344,8 +347,18 @@ fn layout_from_flat_type<'a>(
                             Ok(Layout::Builtin(Builtin::Byte(tag_to_u8)))
                         }
                     } else {
-                        // panic!("TODO handle a tag union with mutliple tags: {:?}", tags);
-                        Ok(Layout::Tag(&[]))
+                        let mut layouts = MutMap::default();
+                        for (tag_name, arguments) in tags {
+                            let mut arg_layouts = Vec::with_capacity_in(arguments.len(), arena);
+
+                            for arg in arguments {
+                                arg_layouts.push(Layout::from_var(arena, arg, subs, pointer_size)?);
+                            }
+
+                            layouts.insert(tag_name, arg_layouts.into_bump_slice());
+                        }
+
+                        Ok(Layout::Union(arena.alloc(layouts)))
                     }
                 }
             }