Electronic noses are a new class of analytical instruments anticipated to have an impact in many areas. They are based around an array of sensors each having a partial specificity and thus producing an odour fingerprint that can be identified by a pattern recognition system. In this review the technologies involved are discussed, giving particular emphasis to the sensors since these form the heart of the systems. The major sensor technologies are described, indicating both their strengths and their weaknesses. Finally, after a brief discussion of data processing technologies, the review ends with an assessment of the current state of the art and possible future directions.