diff --git a/src/assembler.c b/src/assembler.c
index cbad7c6..8d4bee6 100644
--- a/src/assembler.c
+++ b/src/assembler.c
@@ -17,6 +17,37 @@
 #define IS_LABEL(line) (line->data[line->length - 1] == ':')
 
 /*
+    Add a given line, representing a label, to the symbol table.
+
+    Return NULL on success and an ErrorInfo object on failure (in the case of
+    duplicate labels).
+*/
+static ErrorInfo* add_label_to_table(
+    ASMSymbolTable *symtable, const ASMLine *line, size_t offset)
+{
+    char *symbol = strndup(line->data, line->length - 1);
+    if (!symbol)
+        OUT_OF_MEMORY()
+
+    const ASMSymbol *current = asm_symtable_find(symtable, symbol);
+    if (current) {
+        ErrorInfo *ei = error_info_create(line, ET_LAYOUT, ED_LYT_DUPE_LABELS);
+        error_info_append(ei, current->line);
+        return ei;
+    }
+
+    ASMSymbol *label = malloc(sizeof(ASMSymbol));
+    if (!label)
+        OUT_OF_MEMORY()
+
+    label->offset = offset;
+    label->symbol = symbol;
+    label->line = line;
+    asm_symtable_insert(symtable, label);
+    return NULL;
+}
+
+/*
     Parse an instruction encoded in line into an ASMInstruction object.
 
     On success, return NULL and store the instruction in *inst_ptr. On failure,
@@ -44,7 +75,9 @@ static ErrorInfo* tokenize(AssemblerState *state)
     if (!overlap_table)
         OUT_OF_MEMORY()
 
-    // TODO: fill overlap table for header with pointers to a dummy object
+    ASMLine header_indicator;
+    for (size_t i = 0; i < HEADER_SIZE; i++)
+        overlap_table[state->header.offset + i] = &header_indicator;
 
     ErrorInfo *ei = NULL;
     ASMInstruction dummy = {.next = NULL}, *inst, *prev = &dummy;
@@ -77,7 +110,8 @@ static ErrorInfo* tokenize(AssemblerState *state)
             }
         }
         else if (IS_LABEL(line)) {
-            // TODO: add to symbol table
+            if ((ei = add_label_to_table(state->symtable, line, offset)))
+                goto cleanup;
         }
         else {
             if ((ei = parse_instruction(line, &inst, offset)))
diff --git a/src/assembler/errors.c b/src/assembler/errors.c
index 172e558..67ece6d 100644
--- a/src/assembler/errors.c
+++ b/src/assembler/errors.c
@@ -33,6 +33,7 @@ static const char *asm_error_descs[] = {
 
     "header offset exceeds given ROM size",             // ED_LYT_HEADER_RANGE
     "declared ROM size in header exceeds actual size",  // ED_LYT_DECLARE_RANGE
+    "duplicate definitions for label",                  // ED_LYT_DUPE_LABELS
     "location overlaps with ROM header",                // ED_LYT_HEAD_OVERLAP
     "location overlaps with previous instruction",      // ED_LYT_INST_OVERLAP
     "location overlaps with previous data",             // ED_LYT_DATA_OVERLAP
diff --git a/src/assembler/errors.h b/src/assembler/errors.h
index f912590..87c3cf5 100644
--- a/src/assembler/errors.h
+++ b/src/assembler/errors.h
@@ -29,6 +29,7 @@ typedef enum {
 
     ED_LYT_HEADER_RANGE,
     ED_LYT_DECLARE_RANGE,
+    ED_LYT_DUPE_LABELS,
     ED_LYT_HEAD_OVERLAP,
     ED_LYT_INST_OVERLAP,
     ED_LYT_DATA_OVERLAP,
diff --git a/src/assembler/state.c b/src/assembler/state.c
index 77a8eb4..7f6766f 100644
--- a/src/assembler/state.c
+++ b/src/assembler/state.c
@@ -128,6 +128,40 @@ void asm_symtable_free(ASMSymbolTable *symtable)
     free(symtable);
 }
 
+/*
+    ...
+*/
+static inline size_t hash_key(const char *key)
+{
+    return 0;
+}
+
+/*
+    Search for a key in the symbol table.
+
+    Return the key on success and NULL on failure.
+*/
+const ASMSymbol* asm_symtable_find(const ASMSymbolTable *tab, const char *key)
+{
+    ASMSymbol *symbol = tab->buckets[hash_key(key)];
+    while (symbol) {
+        if (!strcmp(key, symbol->symbol))
+            return symbol;
+        symbol = symbol->next;
+    }
+    return NULL;
+}
+
+/*
+    Insert a symbol into the table.
+*/
+void asm_symtable_insert(ASMSymbolTable *tab, ASMSymbol *symbol)
+{
+    size_t index = hash_key(symbol->symbol);
+    symbol->next = tab->buckets[index];
+    tab->buckets[index] = symbol;
+}
+
 #ifdef DEBUG_MODE
 /*
     DEBUG FUNCTION: Print out an ASMLine list to stdout.
diff --git a/src/assembler/state.h b/src/assembler/state.h
index b922059..fe98c92 100644
--- a/src/assembler/state.h
+++ b/src/assembler/state.h
@@ -53,6 +53,7 @@ typedef struct ASMData ASMData;
 struct ASMSymbol {
     size_t offset;
     char *symbol;
+    const ASMLine *line;
     struct ASMSymbol *next;
 };
 typedef struct ASMSymbol ASMSymbol;
@@ -92,6 +93,9 @@ void asm_instructions_free(ASMInstruction*);
 void asm_data_free(ASMData*);
 void asm_symtable_free(ASMSymbolTable*);
 
+const ASMSymbol* asm_symtable_find(const ASMSymbolTable*, const char*);
+void asm_symtable_insert(ASMSymbolTable*, ASMSymbol*);
+
 #ifdef DEBUG_MODE
 void asm_lines_print(const ASMLine*);
 #endif