diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 5a9a232..acaf967 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -1,5 +1,6 @@
 add_library(imp
   am.c
+  env.c
   memory_stream.c
   parse.c
   store.c
diff --git a/lib/env.c b/lib/env.c
new file mode 100644
index 0000000..272bb52
--- /dev/null
+++ b/lib/env.c
@@ -0,0 +1,80 @@
+#include "env.h"
+
+#include <assert.h>
+#include <string.h>
+
+static bool symbol_eq(symbol_t *s, symbol_t *t)
+{
+	return s->len == t->len && memcmp(s->buf, t->buf, s->len) == 0;
+}
+
+static expr_t **lookup(am_t *am, bool *found)
+{
+	assert(am->expr != NULL);
+	assert(am->expr->is_atom);
+	assert(am->expr->atom.type == ATOM_TYPE_SYMBOL);
+
+	if (am->env->is_atom) {
+		assert(am->env->atom.type == ATOM_TYPE_EMPTY_LIST);
+		*found = false;
+		return &am->env;
+	}
+
+	expr_t *prev;
+	for (expr_t *list = am->env; !list->is_atom; list = list->pair.cdr) {
+		assert(list != NULL);
+
+		expr_t *entry = list->pair.car;
+		assert(!entry->is_atom);
+
+		expr_t *key = entry->pair.car;
+		assert(key != NULL);
+		assert(key->is_atom);
+		assert(key->atom.type == ATOM_TYPE_SYMBOL);
+
+		if (symbol_eq(&am->expr->atom.symbol, &key->atom.symbol)) {
+			*found = true;
+			return &entry->pair.cdr;
+		}
+
+		prev = list;
+	}
+
+	assert(!prev->is_atom);
+	assert(prev->pair.cdr->is_atom);
+	assert(prev->pair.cdr->atom.type == ATOM_TYPE_EMPTY_LIST);
+	*found = false;
+	return &prev->pair.cdr;
+}
+
+void env_init(am_t *am, store_t *store)
+{
+	am->env = store_alloc(store);
+	am->env->is_atom = true;
+	am->env->atom.type = ATOM_TYPE_EMPTY_LIST;
+}
+
+void env_fetch(am_t *am)
+{
+	bool found;
+	expr_t *val = *lookup(am, &found);
+	am->val = found ? val : NULL;
+}
+
+void env_set(am_t *am, store_t *store)
+{
+	bool found;
+	expr_t **loc = lookup(am, &found);
+	if (found) {
+		*loc = am->val;
+	} else {
+		(*loc)->is_atom = false;
+		(*loc)->pair.cdr = store_alloc(store);
+		(*loc)->pair.cdr->is_atom = true;
+		(*loc)->pair.cdr->atom.type = ATOM_TYPE_EMPTY_LIST;
+
+		expr_t *entry = (*loc)->pair.car = store_alloc(store);
+		entry->pair.car = am->expr;
+		entry->pair.cdr = am->val;
+	}
+}
diff --git a/lib/include/am.h b/lib/include/am.h
index eb96ddd..97557e2 100644
--- a/lib/include/am.h
+++ b/lib/include/am.h
@@ -6,7 +6,7 @@
 #define AM_STACK_SIZE 128U
 
 typedef struct {
-	expr_t *expr;
+	expr_t *env, *expr, *val;
 	expr_t **sp, *stack[AM_STACK_SIZE];
 } am_t;
 
diff --git a/lib/include/env.h b/lib/include/env.h
new file mode 100644
index 0000000..d263bb3
--- /dev/null
+++ b/lib/include/env.h
@@ -0,0 +1,11 @@
+#ifndef ENV_H
+#define ENV_H
+
+#include "am.h"
+#include "store.h"
+
+void env_init(am_t *am, store_t *store);
+void env_fetch(am_t *am);
+void env_set(am_t *am, store_t *store);
+
+#endif
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 051a173..b49d768 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -10,6 +10,7 @@ endfunction()
 
 add_test_suites(
   am_tests.c
+  env_tests.c
   parse_tests.c
   store_tests.c
   token_tests.c
diff --git a/tests/env_tests.c b/tests/env_tests.c
new file mode 100644
index 0000000..535eeb8
--- /dev/null
+++ b/tests/env_tests.c
@@ -0,0 +1,78 @@
+#include "env.h"
+#include "unity.h"
+
+#include <string.h>
+
+static am_t am;
+static store_t store;
+
+static expr_t *integer(int64_t value)
+{
+	expr_t *expr = store_alloc(&store);
+	expr->is_atom = true;
+	expr->atom.type = ATOM_TYPE_INTEGER;
+	expr->atom.integer = value;
+	return expr;
+}
+
+static expr_t *symbol(const char *s)
+{
+	expr_t *expr = store_alloc(&store);
+	expr->is_atom = true;
+	expr->atom.type = ATOM_TYPE_SYMBOL;
+	memcpy(expr->atom.symbol.buf, s, strlen(s));
+	return expr;
+}
+
+void setUp(void)
+{
+	am_init(&am);
+	store_init(&store);
+	env_init(&am, &store);
+}
+
+void tearDown(void)
+{
+}
+
+static void test_set_foo_to_42_then_fetch(void)
+{
+	am.expr = symbol("foo");
+	am.val = integer(42);
+	env_set(&am, &store);
+
+	am.expr = symbol("foo");
+	am.val = NULL;
+	env_fetch(&am);
+
+	TEST_ASSERT_NOT_NULL(am.val);
+	TEST_ASSERT_TRUE(am.val->is_atom);
+	TEST_ASSERT_EQUAL(ATOM_TYPE_INTEGER, am.val->atom.type);
+	TEST_ASSERT_EQUAL(42, am.val->atom.integer);
+}
+
+static void test_update_foo_from_123_to_456_then_fetch(void)
+{
+	am.expr = symbol("foo");
+	am.val = integer(123);
+	env_set(&am, &store);
+	am.val = integer(456);
+	env_set(&am, &store);
+
+	am.expr = symbol("foo");
+	am.val = NULL;
+	env_fetch(&am);
+
+	TEST_ASSERT_NOT_NULL(am.val);
+	TEST_ASSERT_TRUE(am.val->is_atom);
+	TEST_ASSERT_EQUAL(ATOM_TYPE_INTEGER, am.val->atom.type);
+	TEST_ASSERT_EQUAL(456, am.val->atom.integer);
+}
+
+int main(void)
+{
+	UNITY_BEGIN();
+	RUN_TEST(test_set_foo_to_42_then_fetch);
+	RUN_TEST(test_update_foo_from_123_to_456_then_fetch);
+	return UNITY_END();
+}