The main motivation for applying capacity planning techniques to the design of Internet-based video services is that very often these systems incur significant delays that decrease the offered Quality of Service below an acceptable level. However, the very high fluctuations of the Internet traffic make the identification of the correct capacity of the server and network components a very hard task to be accomplished. In this paper we analyze the metrics used to evaluate the performance perceived by users of video streaming applications and we introduce and validate some new metrics. These variables may be used to parameterize the models required by capacity planning studies. A characterization of users’ behavior for live and stored video streams is presented. The knowledge of the behavior of these two streams is important to identify the capacity to be allocated in order to satisfy the QoS requirements. Several issues concerning the security of video streaming are also discussed...