The chilled mirror hygrometer (CMH) has been used as a NIST-referenced humidity transfer standard in metrology laboratories for the past four decades. Its principle of operation is simple and elegant. Its totally inert construction, wide measuring span, and high precision have led to thousands of industrial control installations. The proper application of any process monitoring device, and especially a humidity sensor, first requires a good understanding of how the sensor works and its advantages as well as its limitations. That is what this article is all about.