Compound interest is often called the eighth wonder of the world — and for good reason. Unlike simple interest, which only earns returns on your original principal, compound interest earns returns on both your principal and the accumulated interest from previous periods. Over time, this exponential growth transforms modest savings into significant wealth. This guide explains exactly how compound interest works, the formula behind it, the critical role of compounding frequency, and why starting early is the single most powerful financial decision you can make.