In this chapter after introducing the basics of measurements in Section 1.1, the characterisation and classifications of sensors are described in Section 1.2. Principles of operation and applications of four groups of sensors are included. The first group is common sensors, the majority of which are the well-established solid state sensors routinely used in a wide range of industrial applications. For clarity of presentation, these sensors are identified in accordance with their input measurand as chemical, mechanical, magnetic, biological and optical sensors. The next three groups are identified in relation to their industrial applications as environmental, healthcare and automotive sensors. Classification in a more precise manner would necessitate undesirable inclusion of a huge number of other sensors. The purpose of Section 1.3 is twofold: first, to review selected classes of advanced sensors that offer significant scope for commercial exploitation, and secondly to set the scene for the next six chapters, which focus on industrially important areas of fibre optic and solid state sensors. Advanced sensors, challenges and opportunities are described in Sections 1.4 and 1.5, respectively. This chapter concludes with a review of the remaining six chapters.