This book addresses issues concerning the engineering of system prod­ ucts that make use of computing technology. These systems may be prod­ ucts in their own right, for example a computer, or they may be the computerised control systems inside larger products, such as factory automation systems, transportation systems and vehicles, and personal appliances such as portable telephones. In using the term engineering the authors have in mind a development process that operates in an integrated sequence of steps, employing defined techniques that have some scientific basis. Furthermore we expect the operation of the stages to be subject to controls and standards that result in a product fit for its intended purpose, both in the hands of its users and as a business venture. Thus the process must take account of a wide range of requirements relating to function, cost, size, reliabili­ ty and so on. It is more difficult to define the meaning of computing technology. These days this involves much more than computers and software. For example, many tasks that might be performed by software running in a general purpose computer can also be performed directly by the basic technology used to construct a computer, namely digital hardware. However, hardware need not always be digital; we live in an analogue world, hence analogue signals appear on the boundaries of our systems and it can sometimes be advantageous to allow them to penetrate further.