diff --git a/Makefile b/Makefile
index 840f356..c51c00f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,11 @@
 CC = gcc
 LD = $(CC)
-#PFLAGS = -pg
-PFLAGS += -O3
+PFLAGS = -g
+#PFLAGS += -O3
 #PFLAGS += -DDEBUG_MAPPER
 #PFLAGS += -DDEBUG_RENDER
 #PFLAGS += -DDEBUG_PPU -DDEBUG_VRAM -DDEBUG_OAM
+PFLAGS += -DDEBUG_APU
 #PFLAGS += -DE6502_DEBUG
 CFLAGS = $(PFLAGS) -Wall -Werror -Wshadow -Wunused -I..
 LDFLAGS = $(PFLAGS)
@@ -23,11 +24,12 @@ SRC_SRCS_1 = nese.c ines.c
 SRC_SRCS_1 += nes.c ppu.c input.c
 SRC_SRCS_1 += cart.c mapper.c
 SRC_SRCS_1 += sdl_render.c sdl_input.c
+SRC_SRCS_1 += apu.c
 
 MAPDIR = src/map
 MAP_SRCS_1 = nrom.c mmc1.c cnrom.c mmc3.c
 
-EXT_SRCS_1 = e6502/e6502.c e6502/opcodes.c
+EXT_SRCS_1 = e6502/e6502.c e6502/opcodes.c blip-buf/blip_buf.c
 
 
 SRCS_1 = $(SRC_SRCS_1:%=$(SRCDIR)/%)
diff --git a/src/apu.c b/src/apu.c
new file mode 100644
index 0000000..bd34c17
--- /dev/null
+++ b/src/apu.c
@@ -0,0 +1,259 @@
+#ifndef APU_H_
+#define APU_H_
+
+#include "apu.h"
+#include "blip-buf/blip_buf.h"
+
+/*
+static const uint8_t length_lut [32] = {
+    0x0A, 0xFE, 0x14, 0x02, 0x28, 0x04, 0x50, 0x06,
+    0xA0, 0x08, 0x3C, 0x0A, 0x0E, 0x0C, 0x1A, 0x0E,
+    0x0C, 0x10, 0x18, 0x12, 0x30, 0x14, 0x60, 0x16,
+    0xC0, 0x18, 0x48, 0x1A, 0x10, 0x1C, 0x20, 0x1E,
+};
+*/
+
+static const uint16_t dmc_period_lut [16] = {
+    428, 380, 340, 320, 286, 254, 226, 214,
+    190, 160, 142, 128, 106,  84,  72,  54,
+};
+
+static void nes_apu_write_square(nes_apu_Channel* channel,
+                                 int reg, uint8_t val) {
+    APU_LOG("APU: Square %d < %02x\n", reg, val);
+    // TODO
+}
+
+static void nes_apu_write_triangle(nes_apu_Channel* channel,
+                                   int reg, uint8_t val) {
+    APU_LOG("APU: Triangle %d < %02x\n", reg, val);
+    // TODO
+}
+
+static void nes_apu_write_noise(nes_apu_Channel* channel,
+                                int reg, uint8_t val) {
+    APU_LOG("APU: Noise %d < %02x\n", reg, val);
+    // TODO
+}
+
+static void nes_apu_write_dmc(nes_apu_Channel* channel,
+                              int reg, uint8_t val) {
+    APU_LOG("APU: DMC %d < %02x\n", reg, val);
+
+    if (reg == 0) {
+        channel->period = dmc_period_lut[val & 0xFU];
+        if (!(val & apu_DMC_IRQ_Enable)) {
+            channel->interrupt = 0;
+        }
+
+    } else if (reg == 1) {
+        channel->sample = (val & 0x7FU);
+
+    } else if (reg == 3) {
+        channel->length = val;
+    }
+}
+
+int nes_apu_init(nes_apu* apu, int clock, int frequency,
+                 uint8_t(*mem_read)(void*, uint16_t),
+                 void* arg_mem) {
+    int ret = 0;
+
+    // 20 ms buffer
+    apu->blip = blip_new(frequency / 50);
+
+    if (NULL == apu->blip) {
+        APU_ERR("APU: Failed to create resampler\n");
+        ret = -1;
+
+    } else {
+        blip_set_rates(apu->blip, clock, frequency);
+
+        nes_apu_reset(apu);
+
+        apu->mem_read = mem_read;
+        apu->arg_mem = arg_mem;
+
+        apu->channels[apu_Channel_Square_0].write = nes_apu_write_square;
+        apu->channels[apu_Channel_Square_1].write = nes_apu_write_square;
+        apu->channels[apu_Channel_Triangle].write = nes_apu_write_triangle;
+        apu->channels[apu_Channel_Noise].write    = nes_apu_write_noise;
+        apu->channels[apu_Channel_DMC].write      = nes_apu_write_dmc;
+
+        apu->frame_lfsr = 0x7FFFU;
+        apu->frame_period = clock / 240;
+    }
+
+    return ret;
+}
+
+void nes_apu_done(nes_apu* apu) {
+    blip_delete(apu->blip);
+}
+
+void nes_apu_reset(nes_apu* apu) {
+    apu->status = 0;
+
+    for (int chan = 0; chan < nes_apu_chan_count; ++chan) {
+        apu->channels[chan].length = 0;
+    }
+
+    apu->channels[apu_Channel_Triangle].sample = 15;
+    apu->channels[apu_Channel_DMC].reg[1] &= 1;
+    apu->channels[apu_Channel_DMC].sample = 0;
+}
+
+uint8_t nes_apu_read(nes_apu* apu, uint16_t addr) {
+    uint8_t val = 0;
+
+    if (addr == nes_apu_reg_status) {
+        for (int chan = 0; chan < nes_apu_chan_count; ++chan) {
+            if (apu->channels[chan].length > 0) {
+                val |= (1 << chan);
+            }
+        }
+
+        if (apu->channels[apu_Channel_DMC].interrupt) {
+            val |= apu_Status_DMC_Int;
+        }
+
+        if (apu->status & apu_Status_Frame_Int) {
+            val |= apu_Status_Frame_Int;
+            apu->status &= ~apu_Status_Frame_Int;
+        }
+
+        APU_LOG("APU: Status %02x\n", val);
+    }
+
+    return val;
+}
+
+void nes_apu_write(nes_apu* apu, uint16_t addr, uint8_t val) {
+    if (addr < nes_apu_reg_base +
+               (nes_apu_chan_count * nes_apu_chan_size)) {
+        int chan = (addr - nes_apu_reg_base) / nes_apu_chan_size;
+        int reg = (addr - nes_apu_reg_base) % nes_apu_chan_size;
+
+        nes_apu_Channel* channel = &apu->channels[chan];
+        channel->write(channel, reg, val);
+        channel->reg[reg] = val;
+
+    } else if (addr == nes_apu_reg_status) {
+        APU_LOG("APU: Enable %02x\n", val);
+
+        for (int chan = 0; chan < nes_apu_chan_count; ++chan) {
+            if (!((val >> chan) & 1)) {
+                apu->channels[chan].length = 0;
+            }
+            val >>= 1;
+        }
+
+        if (    (val & (1 << apu_Channel_DMC)) &&
+                !(apu->status & (1 << apu_Channel_DMC))) {
+            // TODO: Start DMC
+        }
+        if (!(val & (1 << apu_Channel_DMC))) {
+            apu->channels[apu_Channel_DMC].interrupt = 0;
+        }
+
+        apu->status &= ~((1 << nes_apu_chan_count) - 1);
+        apu->status |= (val & ((1 << nes_apu_chan_count) - 1));
+
+    } else if (addr == nes_apu_reg_frame) {
+        APU_LOG("APU: Frame %02x\n", val);
+
+        apu->frame_reg = val &
+                         (apu_Frame_Mode | apu_Frame_Inhibit);
+
+        apu->frame = 0;
+        apu->frame_delay = 0;
+
+        if (val & apu_Frame_Inhibit) {
+            apu->status &= ~apu_Status_Frame_Int;
+        }
+    }
+}
+
+nes_apu_Result nes_apu_run(nes_apu* apu, int cycles) {
+/*
+    nes_apu_run_dmc(&apu->channels[apu_Channel_DMC],
+                    apu->blip, cycles);
+*/
+    while (cycles > 0) {
+        int run = cycles;
+        if (run > apu->frame_delay) {
+            run = apu->frame_delay;
+        }
+/*
+        nes_apu_run_square(&apu->channels[apu_Channel_Square_0],
+                           apu->blip, run);
+        nes_apu_run_square(&apu->channels[apu_Channel_Square_1],
+                           apu->blip, run);
+        nes_apu_run_triangle(&apu->channels[apu_Channel_Triangle],
+                             apu->blip, run);
+        nes_apu_run_noise(&apu->channels[apu_Channel_Noise],
+                          apu->blip, run);
+*/
+        apu->frame_delay -= run;
+
+        if (apu->frame_delay <= 0) {
+            int end = 0;
+            int clock = 1;
+            int half_clock = 0;
+
+            apu->frame_delay += apu->frame_period;
+
+            if (1 == apu->frame) {
+                half_clock = 1;
+            } else if (0 == apu->frame && 0 == apu->frame_reg) {
+                apu->status |= apu_Status_Frame_Int;
+            }
+
+            if (apu->frame_reg & apu_Frame_Mode) {
+                if (3 == apu->frame) {
+                    clock = 0;
+                } else if (4 == apu->frame) {
+                    half_clock = 1;
+                    end = 1;
+                }
+            } else {
+                if (3 == apu->frame) {
+                    half_clock = 1;
+                    end = 1;
+                }
+            }
+
+            if (half_clock) {
+/*
+                nes_apu_clock_length(&apu->channels[apu_Channel_Square_0], 32);
+                nes_apu_clock_length(&apu->channels[apu_Channel_Square_1], 32);
+                nes_apu_clock_length(&apu->channels[apu_Channel_Triangle], 128);
+                nes_apu_clock_length(&apu->channels[apu_Channel_Noise], 32);
+
+                nes_apu_clock_sweep(&apu->channels[apu_Channel_Square_0], -1);
+                nes_apu_clock_sweep(&apu->channels[apu_Channel_Square_1], 0);
+*/
+            }
+
+            if (clock) {
+/*
+                nes_apu_clock_envelope(&apu->channels[apu_Channel_Square_0]);
+                nes_apu_clock_envelope(&apu->channels[apu_Channel_Square_1]);
+                nes_apu_clock_linear(&apu->channels[apu_Channel_Triangle]);
+                nes_apu_clock_envelope(&apu->channels[apu_Channel_Noise]);
+*/
+            }
+
+            if (end) apu->frame = 0;
+            else apu->frame++;
+        }
+
+        cycles -= run;
+    }
+
+    return (    (apu->status & apu_Status_Frame_Int) ||
+                apu->channels[apu_Channel_DMC].interrupt ) ?
+            apu_Result_IRQ : apu_Result_Running;
+}
+
+#endif // APU_H_
diff --git a/src/apu.h b/src/apu.h
index e1a5970..07c5f22 100644
--- a/src/apu.h
+++ b/src/apu.h
@@ -2,17 +2,145 @@
 #define ENES_APU_H_
 
 #include <stdint.h>
+#include <stdio.h>
 
 
-// TODO: Stubbed
+#define DBG_LOG(...) printf(__VA_ARGS__)
+#define ERR_LOG(...) DBG_LOG(__VA_ARGS__)
 
-typedef struct {
+#ifdef DEBUG_APU
+#define APU_LOG DBG_LOG
+#else
+#define APU_LOG(...)
+#endif
+#define APU_ERR ERR_LOG
+
+
+#define nes_apu_frame_period = 7450;
+
+typedef enum {
+    apu_Status_Square_0  = 0b00000001,
+    apu_Status_Square_1  = 0b00000010,
+    apu_Status_Triangle  = 0b00000100,
+    apu_Status_Noise     = 0b00001000,
+    apu_Status_DMC       = 0b00010000,
+    apu_Status_Frame_Int = 0b01000000,
+    apu_Status_DMC_Int   = 0b10000000,
+} nes_apu_Status;
+
+typedef enum {
+    apu_Frame_Inhibit = 0b01000000,
+    apu_Frame_Mode    = 0b10000000,
+} nes_apu_Frame;
+
+typedef enum {
+    apu_Channel_Square_0 = 0,
+    apu_Channel_Square_1 = 1,
+    apu_Channel_Triangle = 2,
+    apu_Channel_Noise    = 3,
+    apu_Channel_DMC      = 4,
+} nes_apu_Channel_Index;
+
+// Applies to Square and Noise
+typedef enum {
+    apu_Square_Volume   = 0b00001111,
+    apu_Square_Constant = 0b00010000,
+    apu_Square_Halt     = 0b00100000,
+    apu_Square_Duty     = 0b11000000,
+} nes_apu_Envelope_Reg_0_Mask;
+
+typedef enum {
+    apu_Square_Shift    = 0b00000111,
+    apu_Square_Negate   = 0b00001000,
+    apu_Square_Period   = 0b01110000,
+    apu_Square_Enable   = 0b10000000,
+} nes_apu_Square_Reg_1_Mask;
+
+typedef enum {
+    apu_Triangle_Count  = 0b01111111,
+    apu_Triangle_Halt   = 0b10000000,
+} nes_apu_Triangle_Reg_0_Mask;
+
+typedef enum {
+    apu_Noise_Period    = 0b00001111,
+    apu_Noise_Loop      = 0b10000000,
+} nes_apu_Noise_Reg_2_Mask;
+
+typedef enum {
+    apu_Noise_Length    = 0b11111000,
+} nes_apu_Noise_Reg_3_Mask;
 
+typedef enum {
+    apu_DMC_Period      = 0b00001111,
+    apu_DMC_Loop        = 0b01000000,
+    apu_DMC_IRQ_Enable  = 0b10000000,
+} nes_apu_DMC_Reg_0_Mask;
+
+static inline int nes_apu_osc_timer(uint8_t reg[4]) {
+    return ((unsigned)reg[2] | (((unsigned)reg[3] & 7U) << 8));
+}
+
+static inline int nes_apu_osc_length(uint8_t reg[4]) {
+    return (reg[3] >> 3);
+}
+
+typedef struct nes_apu_Channel_t {
+    uint8_t reg[4];
+    void(*write)(struct nes_apu_Channel_t*, int reg, uint8_t);
+    int length;     // Active counter
+    int next_delta; // APU cycles until next state change
+    int sample;     // Current sample
+
+    union {
+        // Square/Triangle
+        struct {
+            int phase;
+            int sweep;
+            int envelope;
+        };
+        // DMC
+        struct {
+            uint8_t interrupt;
+            uint8_t mask;
+            uint16_t addr;
+            int period;
+        };
+    };
+} nes_apu_Channel;
+
+typedef struct {
+    nes_apu_Channel channels[5];
+    uint8_t (*mem_read)(void*, uint16_t);
+    void* arg_mem;
+    struct blip_t* blip;
+    nes_apu_Status status;
+    uint8_t frame_reg;
+    uint16_t frame_lfsr;
+    int frame;
+    int frame_delay;
+    int frame_period;
 } nes_apu;
 
-static inline uint8_t nes_apu_read(nes_apu*, uint16_t addr) { return 0; }
-static inline void nes_apu_write(nes_apu*, uint16_t addr, uint8_t) {}
-static inline void nes_apu_reset(nes_apu*) {}
+typedef enum {
+    apu_Result_Running  = 0U,
+    apu_Result_IRQ,
+} nes_apu_Result;
+
+#define nes_apu_reg_base    (0x4000U)
+#define nes_apu_chan_count  (5U)
+#define nes_apu_chan_size   (4U)
+#define nes_apu_reg_status  (0x4015U)
+#define nes_apu_reg_frame   (0x4017U)
+
+int nes_apu_init(nes_apu*, int clock, int frequency,
+                 uint8_t(*)(void*, uint16_t), void*);
+void nes_apu_done(nes_apu*);
+void nes_apu_reset(nes_apu*);
+
+uint8_t nes_apu_read(nes_apu*, uint16_t addr);
+void nes_apu_write(nes_apu*, uint16_t addr, uint8_t val);
+
+nes_apu_Result nes_apu_run(nes_apu*, int cycles);
 
 
 #endif // ENES_APU_H_
diff --git a/src/nes.c b/src/nes.c
index 555717e..c39cda4 100644
--- a/src/nes.c
+++ b/src/nes.c
@@ -63,11 +63,18 @@ static void nes_irq(void* sys, int active) {
     e6502_set_irq(&((nes*)sys)->cpu, active);
 }
 
-int nes_init(nes* sys) {
+int nes_init(nes* sys, int audio_freq) {
     e6502_init(&sys->cpu, (e6502_Read*)nes_mem_read,
                (e6502_Write*)nes_mem_write, sys);
     nes_map_set_irq(sys->cart.mapper, nes_irq, sys);
-    return nes_ppu_init(&sys->ppu, &sys->cart);
+    nes_ppu_init(&sys->ppu, &sys->cart);
+    return nes_apu_init(
+            &sys->apu,
+            nes_clock_master_num / nes_clock_master_den,
+            audio_freq,
+            (uint8_t(*)(void*, uint16_t))nes_mem_read,
+            sys
+        );
 }
 
 void nes_reset(nes* sys) {
@@ -83,6 +90,13 @@ nes_ppu_Result nes_step(nes* sys, int* run) {
 
     if (cpu_run > 0) {
         int master_cycles = cpu_run * nes_clock_cpu_div;
+
+        nes_apu_Result aresult = nes_apu_run(&sys->apu,
+                                             master_cycles);
+
+        // TODO: Does this conflict with MMC3?
+        e6502_set_irq(&sys->cpu, aresult == apu_Result_IRQ);
+
         int ppu_cycles = master_cycles / nes_clock_ppu_div;
         result = nes_ppu_run(&sys->ppu, ppu_cycles);
 
diff --git a/src/nes.h b/src/nes.h
index 5f13176..0b16945 100644
--- a/src/nes.h
+++ b/src/nes.h
@@ -13,6 +13,7 @@
 #define nes_clock_master_den    (44U)
 #define nes_clock_cpu_div       (12U)
 #define nes_clock_ppu_div       (4U)
+#define nes_clock_apu_div       (24U)
 
 #define nes_chr_page_size       (0x1000U)
 #define nes_vram_page_size      (0x0400U)
@@ -55,7 +56,7 @@ uint8_t nes_mem_read(nes*, uint16_t addr);
 
 void nes_mem_write(nes*, uint16_t addr, uint8_t);
 
-int nes_init(nes*);
+int nes_init(nes*, int audio_freq);
 
 void nes_reset(nes*);
 
diff --git a/src/nese.c b/src/nese.c
index 70f7b0d..80f1ca4 100644
--- a/src/nese.c
+++ b/src/nese.c
@@ -7,6 +7,8 @@
 #include "input.h"
 
 
+#define audio_freq  (44100U)
+
 /*
 void e6502_print_registers(const e6502_Registers* regs,
                            FILE* file) {
@@ -81,7 +83,7 @@ int main(int argc, char* argv[]) {
     }
 
     if (status == 0) {
-        status = nes_init(&sys);
+        status = nes_init(&sys, audio_freq);
     }
 
     if (status == 0) {