1 Introduction Worldwide, sensor development is a fast growing discipline. Today’s sensor market offers thousands of sensor types, for almost every measurable quantity, for a broad area of applications, and with a wide diversity in quality. Many research groups are active in the sensor field, exploring new technologies, investigating new principles and structures, aiming at reduced size and price, at the same or even better performance. System engineers have to select the proper sensors for their design, from an overwhelming volume of sensor devices and associated equipment. A well motivated choice requires thorough knowledge of what is available on the market, and a good insight in current sensor research to be able to anticipate forthcoming sensor solutions. This introductory chapter gives a general view on sensors their functionality, the nomenclature and global properties as a prelude to a more in-depth discussion about sensor performance and operation principles. 1.1 Sensors in Mechatronics 1.1.1 Definitions A transducer is an essential part of any information processing system that operates in more than one physical domain. These domains are characterized by the type of quantity that provides the carrier of the relevant information. Examples are the optical, electrical, magnetic, thermal and mechanical domains. A transducer is that part of a measurement system that converts information about a measurand from one domain to another, ideally without information loss. A transducer has at least one input and one output. In measuring instruments, where information processing is performed by electrical signals, either the output or the input is of electrical nature (voltage, current, resistance, capacitance and so on), whereas the other is a non-electrical signal (displacement, temperature, elasticity and so on). A transducer with a non-electrical input is an input transducer, intended to convert a non-electrical quantity into an electrical signal in order to me