With the development of digital rights management systems, new commercial applications for the trade with digital goods will be introduced, and new information services will be provided. As digital goods or services can be delivered over networks, it is also desired that they can immediately be paid electronically. Thus, it is assumed that the trade with digital goods stimulates the deployment of electronic payment systems. Furthermore, new commercial models make new demands on specialized payment systems, e.g., low-value payments should be supported in an economically reasonable way. Meanwhile, there exists a large body of literature on electronic payment system. In this paper, we give a survey of these systems. We point out the requirements they should fulfill and present briefly the basic principles for different categories of payment systems, and consider a few candidates.