Several tools are available for decompiling DLLs. Here are some popular ones:

(For Native DLLs)

If you suspect the DLL is a .NET assembly, you are in luck. The reversal is often painless.

By following this guide, you'll be well on your way to decompiling DLLs and unlocking the secrets of compiled code.

This requires heavy-duty tools. The learning curve is steeper.