The basic aim of this book is to provide an introduction to the principles of industrial control and instrumentation such that the characteristics and basic principles of elements within such systems can be understood, the ways in which instrumentation and control systems are put together can be appreciated, the reasons for the selection of specific components appreciated, and manufacturer's specifications can be interpreted and used. The book is concerned with the basic principles of measurement and control systems and the hardware used. Chapters 1 and 2 give an introduction to the basic concepts and terminology of measurement and control systems. Chapters 4, 5, 6 and 7 offer more detailed discussions of the elements used to build up such systems. Chapters 8 and 9 bring the elements together in a consideration of measurement and control systems. The aim is seen as being an introduction to the topic and thus the book includes only a superficial consideration of time-variant relationships and their associated mathematics. For the most part the level of mathematics required in this book is just a reasonable acquaintance with algebra. In a few instances an elementary knowledge of calculus is desirable. A basic knowledge of physical science and in particular electrical principles has been assumed. The book covers the Business and Technician Education Council unit Industrial Control and Instrumentation U86/345. This unit has been designed for use within BTEC National Certificates and Diploma courses in the areas of Electrical and Electronic Engineering, Electronic Engineering, Communications Engineering and Computer Engineering though it is also recognized that it might be also appropriate for use in other BTEC programmes. The book is also seen as likely to be of use in other courses, indeed any course where a basic appreciation and introduction to instrumentation and control systems is required.