Message Sequence Charts (MSCs) are a technique to describe patterns of interactions between the components of interactive distributed systems by specific diagrams. MSCs have evolved in telecommunication applications and become very popular in the design of software architectures and, generally, of distributed software systems. They are used frequently to describe scenarios of interactions illustrating instances of use cases. Nevertheless, both the semantics of MSCs as a technique of specification and their methodological and technical role in the development process have not been precisely and sufficiently clarified, so far. In this paper, we discuss the systematic application of MSCs in the software development process.