At the beginning of this book, and in the absence of guidance from IUPAC, it is appropriate to clarify the term 'chemical sensor'. A chemical sensor may be defined as a simple-to-use, robust device that is capable of reliable quantitative or qualitative recognition of atomic, molecular or ionic species. It is hard to imagine a field of applied chemistry in which a significant impact could not be made by such a device. Undoubtedly, it is this potential that has fuelled the contemporary preoccupation with chemical sensors. An unfortunate side-effect of this otherwise welcome interest is the use of the term 'chemical sensor' to add the chemical equivalent of a 'High-Tech gloss' to a rather ordinary device, publication, conference or research group. This loose usage of terminology is responsible in part for the ambiguity that surrounds many chemists' concepts of the form and function of chemical sensors. Further ambiguity arises from the extravagant claims that have been made for some sensors, and the impression that has been given of much 'verging-on-a-breakthrough' research. The research chemist engaged in sensor development should be mindful of the fact that the ultimate target for these devices is the real world, and that a successful laboratory device operating under well-defined conditions and careful calibration does not constitute a chemical sensor. Research into chemical sensors is not a recent phenomenon; it has been under way for over 80 years.