Sonic The Hedgehog

At the title screen, press Up, Down, Left, Right. You should hear a noise like a ring being collected. Then, hold A and press Start for a level select!

At the title screen, hold A then press Up, Down, Left, Right. When you hear the chime, press
Start.

If that doesn't work, try this: at the title screen, press Up, Down, Left, Right, hold A, and press
Start. After doing this once, you only need to hold Start + A to enter the level select.

Press Up, Down, Left, Right at the title screen. A chime will confirm correct code entry. Hold A + Start to begin game play. In the Japanese version, press Up, Down(3), Left, Right at the title screen. Hold A + Start to begin game play.

This code is for the Japanese version. At the title screen, press Up, Down, Down, Down, Left, Right. You'll hear a ring sound. Hold A and Press Start for the level select.

In Marble Zone act 3 you will find three chandeliers with spikes in them. Go to the far wall (on
the right) and jump to the right to find some goodies. Now jump through the wall on the right
again, but be careful of the other chandelier.

Using the level select cheat, scroll down and select "Special Stage". Upon completion on the stage, you will receive a chaos emerald, and begin playing your game on the first stage. At this point, RESET the game via the RESET button on your Genesis, re-enter the level select code, and repeat the process. You will enter the next Special Stage level, and have a chance to earn the 2nd Emerald. You can repeat this process over and over until you have all 6 Chaos Emeralds. However, once you have all six, do NOT reset the system and select a level, or else you will lose them all.

On the last level (Scrap Brain), find the ledges that move right then left. Go to the left side of the
wall and wait for the platform to move. As soon as it starts to move left, duck. You'll go through
the wall. While here, press Left to teleport right and Right to teleport to the left.

At the title screen, press C,C,C,C,C,C,Up, Down, Left, Right. When the demo starts, hold A, B, C, Down then press Start. Instead of the Sonic Team logo, you will see a list of the game's developers in Japanese. When the title screen appears, a flashing "Press Start Button" will be there under Sonic's head.

While using the debug mode, enter any special stage. Turn into any object and travel outside the
special stage area. Ou will fall into a secret special stage filled with one-ups and chaos emeralds.

Note: This code is for the Japanese version.
At the title screen, press C, C, Up, Down, Down, Down, Left, Right. You'll hear a ring sound. Pause the game when playing, and hold B to play in Slow Motion, press C to skip a frame, or press A to go back to the Title Screen.