re languages that each cover a certain aspect of system models (interaction patterns, data types, state-orientation). Efforts to merge all three languages in an enhanced SDL are on-going. Note that, although developed by ITU-T, SDL is not specifically designed for telecom systems (the language has no specific support for telecom unique concepts). SDL can be used for general modelling of process control systems. An evolution towards the integration of SDL with UML-RT is also on-going. UML-RT is based on ROOM, see Selic et al. (1994), and is the real-time version of initial UML. While SDL and UML have a history of bottom-up evolution and software sys- tem focus, SM&AML represents a top-down approach (and is therefore more architecture-oriented), with a focus on processing systems in general and telecom systems in specific. The history of SM&AML is almost as long as that for SDL. SM&AML is based on 17 years of research, aimed at finding a precise and effec- tive method for modelling. The history of SM&AML is reflected in the architecture of SM&AML itself. SM&AML consists of three major parts: Concepts, symbols, architectural principles and modelling techniques for net- works and telecom systems modelling, covered primarily by Chaps. 3 and 4. These chapters represent the domain-unique part of SM&AML, which has no direct comparison in other languages. Concepts, symbols, architectural principles and modelling techniques for mod- elling Process Control Systems, covered primarily by Chaps. 6 and 7. This part has a more general applicability, and can, to some extent, be compared with lan- guages such as SDL and UML-RT. Support for modelling distribution in general, specifically for protocol design. This aspect is a major concern for networks and telecom systems, but not restricted to those systems. It is covered primarily by Chap. 5. This component of SM&AML is isolated in order to allow services and functional aspects (a rel- 1.2 Why Modelling Systems Is Important atively static aspect of the architecture of telecom syst