The design of a distributed multimedia application involves both temporal and spatial synchronization problems related to the processing, transport, storage, retrieval and presentation of data, sound, still images and video. Within this general framework, the paper aims at defining a new multimedia synchronization mechanism, the so-called conditional delivery mechanism, which appears particularly useful for achieving both intra- and inter- stream synchronization. Main motivations behind this synchronization mechanism are first discussed on a simple example, namely a distributed application in the area of distance and interactive training, and then the details of the mechanism are presented. This presentation starts with an informal discussion on simple examples, which is followed by a formal specification of the mechanism based on the use of a temporal extension of the Formal Description Technique LOTOS.