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Detecting and classifying odors can be extremely useful. When it comes to food, odors can signal freshness, fermentation levels, even the authenticity of products. Odors help with pollution monitoring, the detection of land mines and other explosives, even the medical diagnostics of breath samples. But it’s not always feasible to use trained human sniffers to get these jobs done. David R. Walt, Shannon E. Stitzel and Matthew J. Aernecke describe the technology behind electronic noses and what they are capable of.