The exchange of goods conducted face-to-face between two parties dates back to before the beginning of recorded history. Traditional means of payment have always had security problems, but now electronic payments retain the same drawbacks and add some risks. Unlike paper, digital `documents' can be copied perfectly and arbitrarily often; digital signatures can be produced by anybody who knows the secret cryptographic key; a buyer's name can be associated with every payment, eliminating the anonymity of cash. Without new security measures, wide-spread electronic commerce is not viable. On the other hand, properly designed electronic payment systems can actually provide better security than traditional means of payments, in addition to flexibility. This article provides an overview of electronic payment systems, focusing on issues related to security.