ss control system. The concept of the processing system is at the very heart of SM&AML, and is reflected by the architecture of SM&AML itself. Section 1.4 presents a high- level functional model of processing systems in general. My experience tells me that people tend to look at the vast amount of documen- tation that is produced in a system development project as a system in itself. It is easy to confuse the system we are designing with the documentation (i.e. the system model) that describes that system. Section 1.5 sheds light on the distinc- tion between the system as such, and its documentation. Designers are familiar with textual design languages for software and hardware, and they manage to design very complex systems by using these types of lan- guages alone. From my experience, there is a certain reluctance among profes- sionals in systems engineering to use modelling languages. It is therefore important to have an understanding of which properties of a modelling language attract engineers who can realize the potential benefits of systems modelling. These issues are discussed in Sect. 1.6. Why Another Modelling Language? The systems that are built, operated and managed today are of an ever-increasing complexity compared to past systems, mostly due to the extraordinary pace of development in electronics, computers and software. Modelling complex systems, their structure, behaviour and characteristics, is therefore becoming an increasingly important part of any systems engineering activity, whether we are developing a telecommunication system, a command and con- trol system, a database management system, a system that controls a manufac- turing process, a computer system or a microprocessor, and whether we do it to provide requirements for systems implementation, or to be able to analyse some system properties, or just to understand the system, or to communicate this understanding in a common language. This is probably also one of the most underrated benefits of systems modelling. Systems modelling technol