Communication protocols are the rules that govern the communication between the different components within a distributed computer system. Since protocols are implemented in software and/or hardware, the question arises whether the existing hardware and software testing methods would be adequate for the testing of communication protocols. The purpose of this paper is to explain in which way the problem of testing protocol implementations is different from the usual problem of software testing. We review the major results in the area of protocol testing and discuss in which way these methods may also be relevant in the more general context of software testing.