An interactive proof system is zero-knowledge if verifiers that interact with the honest prover of the system learn nothing from the interaction beyond the validity of the statement being proved. Classically, every language in NP has a zero-knowledge proof system under certain conditions. In the quantum setting, even finding the general definition is non-trivial. In this talk, we will formally define quantum zero-knowledge proof systems. Then we will discuss some examples of classical zero-knowledge protocols that remain zero-knowledge against quantum attacks.