Sensor systems The dramatic development of sensor technology during the past decade has focused chiefly on the problem of detecting one or more chemicals. In many instances, in both medicine and industry, the problem is to detect small amounts of a chemical in a complex mixture of chemicals. The detection of glucose in blood plasma is an archetypal problem of this kind. Enzyme-based technology has provided elegant solutions to such problems. The high substrate specificity of enzymes has enabled the design and construction of simple, inexpensive instruments which are having a major impact on key, routine analyses in medicine. The specificity of monoclonal antibodies also offers a related solution using the intrinsic properties of natural biosensors. We are interested in solving problems at the opposite end of the spectrum of sensor technology, namely the problem of an integrated measurement of a complex stimulus consisting of some tens to hundreds of different chemicals. The problem of flavour and odour, both of which are key variables in the production of a consistent quality of comestibles, beverages and perfumes, is an example of the problems we have chosen. It is reasonable to enquire about the wisdom of tackling such complex problems at this stage of sensor development. After all, in the fullness of time, biosensor methodology can be expected to advance to a stage at which it is appropriate to address the more complex problems. Our justification for tackling flavour assessment and allied areas is that these important problems are solved daily in a variety of industries through the simple expedient of having a sniff. With a single sniff the sense of smell can reach important commercial decisions such as the fitness of a batch of beer, whisky or wine. Our guiding philosophy is that if we can understand the molecular mechanisms of olfaction we will be able to design and construct an electronic analogue of the biological system. Such devices would, for the first time, provide on-line monitoring of flavours and odours during the manufacturing process. Our understanding of olfactory mechanisms suggests that, in contrast to simpler biosensors, we are dealing with a complex system with many sensors and several levels of information processing. Indeed, the problems may only be soluble in terms of the properties of the system.