diff --git a/src/assembler/parse_util.c b/src/assembler/parse_util.c
index db20736..974a1ad 100644
--- a/src/assembler/parse_util.c
+++ b/src/assembler/parse_util.c
@@ -56,6 +56,34 @@ static bool adjust_for_indirection(ASMArgParseInfo *ap_info)
 }
 
 /*
+    Calculate the mask field for an ASMArgImmediate based on its uval/sval.
+*/
+static void calculate_immediate_mask(ASMArgImmediate *imm)
+{
+    imm->mask = 0;
+    if (imm->sval < 0) {
+        if (imm->sval >= INT8_MIN)
+            imm->mask |= IMM_S8;
+        if (imm->sval >= INT8_MIN + 2)
+            imm->mask |= IMM_REL;
+    } else {
+        imm->mask = IMM_U16;
+        if (imm->uval <= UINT8_MAX)
+            imm->mask |= IMM_U8;
+        if (imm->uval <= INT8_MAX)
+            imm->mask |= IMM_S8;
+        if (imm->uval <= INT8_MAX + 2)
+            imm->mask |= IMM_REL;
+        if (imm->uval <= 7)
+            imm->mask |= IMM_BIT;
+        if (!(imm->uval & ~0x38))
+            imm->mask |= IMM_RST;
+        if (imm->uval <= 2)
+            imm->mask |= IMM_IM;
+    }
+}
+
+/*
     Read in a boolean value and store it in *result.
 */
 bool parse_bool(bool *result, const char *arg, ssize_t size)
@@ -303,12 +331,6 @@ bool argparse_immediate(ASMArgImmediate *result, ASMArgParseInfo ai)
     if (ai.size <= 0)
         return false;
 
-    const ASMDefine *define = asm_deftable_find(ai.deftable, ai.arg, ai.size);
-    if (define) {
-        *result = define->value;
-        return true;
-    }
-
     bool negative = false;
     ssize_t i = 0;
 
@@ -318,9 +340,23 @@ bool argparse_immediate(ASMArgImmediate *result, ASMArgParseInfo ai)
         if (++i >= ai.size)
             return false;
     }
+    ai.arg += i;
+    ai.size -= i;
+
+    const ASMDefine *define = asm_deftable_find(ai.deftable, ai.arg, ai.size);
+    if (define) {
+        if (negative) {
+            result->uval = define->value.uval;
+            result->sval = -define->value.sval;
+            calculate_immediate_mask(result);
+        } else {
+            *result = define->value;
+        }
+        return true;
+    }
 
     uint32_t uval;
-    if (!parse_uint32_t(&uval, ai.arg + i, ai.size - i) || uval > UINT16_MAX)
+    if (!parse_uint32_t(&uval, ai.arg, ai.size) || uval > UINT16_MAX)
         return false;
 
     int32_t sval = negative ? -uval : uval;
@@ -329,28 +365,7 @@ bool argparse_immediate(ASMArgImmediate *result, ASMArgParseInfo ai)
 
     result->uval = uval;
     result->sval = sval;
-    result->mask = 0;
-
-    if (sval < 0) {
-        if (sval >= INT8_MIN)
-            result->mask |= IMM_S8;
-        if (sval >= INT8_MIN + 2)
-            result->mask |= IMM_REL;
-    } else {
-        result->mask = IMM_U16;
-        if (uval <= UINT8_MAX)
-            result->mask |= IMM_U8;
-        if (uval <= INT8_MAX)
-            result->mask |= IMM_S8;
-        if (uval <= INT8_MAX + 2)
-            result->mask |= IMM_REL;
-        if (uval <= 7)
-            result->mask |= IMM_BIT;
-        if (!(uval & ~0x38))
-            result->mask |= IMM_RST;
-        if (uval <= 2)
-            result->mask |= IMM_IM;
-    }
+    calculate_immediate_mask(result);
     return true;
 }
 
@@ -425,11 +440,8 @@ bool argparse_label(ASMArgLabel *result, ASMArgParseInfo ai)
     if (ai.size <= 0 || ai.size >= MAX_SYMBOL_SIZE)
         return false;
 
-    // Validate the label characters:
     for (const char *i = ai.arg; i < ai.arg + ai.size; i++) {
-        char c = *i;
-        if (!((c >= 'a' && c <= 'z') || c == '_' || c == '.' ||
-              (i != ai.arg && c >= '0' && c <= '9')))
+        if (!is_valid_symbol_char(*i, i == ai.arg))
             return false;
     }
 
diff --git a/src/assembler/preprocessor.c b/src/assembler/preprocessor.c
index 7a482b7..8a38314 100644
--- a/src/assembler/preprocessor.c
+++ b/src/assembler/preprocessor.c
@@ -85,15 +85,6 @@
     else FAIL_ON_COND_(true, ED_PP_UNKNOWN)
 
 /*
-    Return whether the given character is a valid label character.
-*/
-static inline bool is_valid_label_char(char c, bool first)
-{
-    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
-           (!first && c >= '0' && c <= '9') || c == '_' || c == '.';
-}
-
-/*
     Functions similar memcpy, but lowercases the characters along the way.
 */
 static void memcpy_lc(char *restrict dst, const char *restrict src, size_t n)
@@ -121,7 +112,7 @@ static size_t read_labels(
         start++;
 
     i = start;
-    while (i < length && is_valid_label_char(source[i], i == start))
+    while (i < length && is_valid_symbol_char(source[i], i == start))
         i++;
 
     if (i == start || i == length || source[i] != ':') {
diff --git a/src/assembler/tokenizer.c b/src/assembler/tokenizer.c
index 042fc15..81bbde2 100644
--- a/src/assembler/tokenizer.c
+++ b/src/assembler/tokenizer.c
@@ -12,6 +12,7 @@
 #include "../logging.h"
 #include "../mmu.h"
 #include "../rom.h"
+#include "../util.h"
 
 /* Internal structs */
 
@@ -128,6 +129,84 @@ static ErrorInfo* add_label_to_table(
 }
 
 /*
+    Handle a define directive by adding an entry to the define table.
+
+    Return NULL on success and an ErrorInfo object on failure.
+*/
+static ErrorInfo* handle_define_directive(
+    const ASMLine *line, ASMDefineTable *deftab)
+{
+    if (!DIRECTIVE_HAS_ARG(line, DIR_DEFINE))
+        return error_info_create(line, ET_PREPROC, ED_PP_NO_ARG);
+
+    size_t start = DIRECTIVE_OFFSET(line, DIR_DEFINE) + 1, i;
+    for (i = start; i < line->length; i++) {
+        if (!is_valid_symbol_char(line->data[i], i == start)) {
+            if (line->data[i] == ' ' && i > start) {
+                i++;
+                break;
+            }
+            return error_info_create(line, ET_PREPROC, ED_PP_BAD_ARG);
+        }
+    }
+
+    if (i >= line->length)  // Missing value for define
+        return error_info_create(line, ET_PREPROC, ED_PP_NO_ARG);
+
+    const char *key = line->data + start;
+    size_t keylen = i - start - 1;
+
+    const ASMDefine *current = asm_deftable_find(deftab, key, keylen);
+    if (current) {
+        ErrorInfo *ei = error_info_create(line, ET_PREPROC, ED_PP_DUPLICATE);
+        error_info_append(ei, current->line);
+        return ei;
+    }
+
+    ASMArgImmediate imm;
+    ASMArgParseInfo info = {
+        .arg = line->data + i, .size = line->length - i, .deftable = deftab};
+    if (!argparse_immediate(&imm, info))
+        return error_info_create(line, ET_PREPROC, ED_PP_BAD_ARG);
+
+    ASMDefine *define = malloc(sizeof(ASMDefine));
+    if (!define)
+        OUT_OF_MEMORY()
+
+    if (!(define->name = strndup(key, keylen)))
+        OUT_OF_MEMORY()
+
+    define->value = imm;
+    define->line = line;
+    asm_deftable_insert(deftab, define);
+    return NULL;
+}
+
+/*
+    Handle an undefine directive by remove an entry in the define table.
+
+    Return NULL on success and an ErrorInfo object on failure.
+*/
+static ErrorInfo* handle_undef_directive(
+    const ASMLine *line, ASMDefineTable *deftab)
+{
+    if (!DIRECTIVE_HAS_ARG(line, DIR_UNDEF))
+        return error_info_create(line, ET_PREPROC, ED_PP_NO_ARG);
+
+    size_t offset = DIRECTIVE_OFFSET(line, DIR_UNDEF) + 1;
+    const char *arg = line->data + offset;
+    size_t size = line->length - offset, i;
+
+    for (i = 0; i < size; i++) {
+        if (!is_valid_symbol_char(arg[i], i == 0))
+            return error_info_create(line, ET_PREPROC, ED_PP_BAD_ARG);
+    }
+
+    asm_deftable_remove(deftab, arg, size);
+    return NULL;
+}
+
+/*
     Handle an origin directive by updating the offset.
 
     Return NULL on success and an ErrorInfo object on failure.
@@ -410,10 +489,12 @@ ErrorInfo* tokenize(AssemblerState *state)
         }
         else if (IS_LOCAL_DIRECTIVE(line)) {
             if (IS_DIRECTIVE(line, DIR_DEFINE)) {
-                // TODO
+                if ((ei = handle_define_directive(line, deftab)))
+                    goto cleanup;
             }
             else if (IS_DIRECTIVE(line, DIR_UNDEF)) {
-                // TODO
+                if ((ei = handle_undef_directive(line, deftab)))
+                    goto cleanup;
             }
             else if (IS_DIRECTIVE(line, DIR_ORIGIN)) {
                 if ((ei = handle_origin_directive(line, &offset)))
diff --git a/src/util.c b/src/util.c
index feb25eb..7c7b409 100644
--- a/src/util.c
+++ b/src/util.c
@@ -54,6 +54,15 @@ uint64_t get_time_ns()
 }
 
 /*
+    Return whether the given character is valid in a symbol (label, define).
+*/
+bool is_valid_symbol_char(char c, bool first)
+{
+    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
+           (!first && c >= '0' && c <= '9') || c == '_' || c == '.';
+}
+
+/*
     Return the name of the region encoded by the given region code.
 
     The given code should not be larger than one nibble. NULL is returned if
diff --git a/src/util.h b/src/util.h
index ad14235..64f21c9 100644
--- a/src/util.h
+++ b/src/util.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #define INVALID_SIZE_CODE 0x8
@@ -12,6 +13,7 @@
 uint8_t bcd_encode(uint8_t);
 uint8_t bcd_decode(uint8_t);
 uint64_t get_time_ns();
+bool is_valid_symbol_char(char, bool);
 const char* region_code_to_string(uint8_t);
 uint8_t region_string_to_code(const char*);
 size_t size_code_to_bytes(uint8_t);