The calorimetric gas sensor has evolved from a simple metal hot wire to a modern device fabricated using micromachining techniques. The principle of operation is to detect the heat evolved from combustible gases on the sensor surface. While the most widely used device is still the discrete catalyst-coated platinum coil known as the pellistor, considerable research has taken place in the last two decades to produce miniature devices that use less power and provide greater measurement capability. These devices have separate heater and temperature control, can be fabricated as arrays, and operated with time-varying signals that produce data that can provide information about chemical composition. The technology for miniaturization of calorimetric devices is also finding new applications in the area of biosensors. This review includes an introduction to calorimetric gas sensors, a review of new thin film structures, catalyst materials, catalyst poisoning effects, new signal operating modes, packaging, biological sensors, and device applications.