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...