Fingerprints are the most common method for criminal identification (and other identification) today. They are useful because fingerprints are unique and don't change as people age. No two people have ever been found to have the same fingerprints but mathematically, it can’t be proven that they are unique. It’s just very, very improbable that two people will have the same prints. The ridges on a finger can be worn down; builders who lay bricks or people who wash dishes will lose some detail. But the ridges grow back. People have tried to remove fingerprints with burns or acid, and that can alter the pattern, but the new pattern is still unique. So it's a pretty handy way to identify people over time.