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Abstract: Gas sensors are employed in many applications including detection of toxic and combustible gases, monitoring emissions from vehicles and other combustion processes, breath analysis for medical diagnosis, and quality control in the chemicals, food and cosmetics industries. Many of these applications employ miniaturized solid-state devices, whose electrical properties change in response to the introduction of chemical analytes into the surrounding gas phase. Key challenges remain as to how to optimize sensor sensitivity, selectivity, speed of response and stability. The principles of operation of such devices vary and a brief review of operating principles based on potentiometric/amperometric, chemisorptive, redox, field effect and nanobalance approaches is presented. Due to simplicity of design and ability to stand up to harsh environments, metal oxide-based chemoresistive devices are commonly selected for these purposes and are therefore the focus of this review. While many studies have been published on the operation of such devices, an understanding of the underlying physicochemical principles behind their operation have trailed behind their technological development. In this article, a detailed review is provided which serves to update progress made along these lines. The introduction of nanodimensioned materials has had a particularly striking impact on the field over the past decade. Advances in materials processing has enabled the fabrication of tailored structures and morphologies offering, at times, orders of magnitude improvements in sensitivity, while high-resolution analytical methods have enabled a much improved examination of the structure and chemistry of these materials. Selected examples, illustrating the type of nanostructured devices being fabricated and tested, are discussed. This review concludes by highlighting trends suggesting directions for future progress. [Copyright &y& Elsevier]