Peggy wants to convince Victor that something is true. But being a very secretive person, she doesn't want to leak any unnecessary information while doing so. Actually, she would like for Victor to learn nothing more that the fact that her something was indeed true. In particular, she doesn't want Victor or anybody listening on their conversation to be able to prove it to anybody else. Zero-Knowledge proofs can achieve just that. We will see different examples, and discuss their applications and limitations. If time allows, we will also see why Trusted Third Parties are overrated.