During the past decade there has been a major change, often described as a paradigm shift, in passive infrared (IR) detection, due to the rapid development of microbolometer IR detectors fabricated by MEMS/MST technology. Microbolometers are now replacing the elegant but costly cryogenically cooled photon detector technology for all but specialised or very high performance applications. Silicon resistance microbolometers were first developed at the Defence Science and Technology Organisation (DSTO), Edinburgh, South Australia, more than two decades ago. In this paper the author introduces a new concept, whereby the microbolometer functions as a transistor. The semiconductor material is silicon or a silicon alloy. The subject matter covers an overview of active bolometer theory, device design and applications. Active operation offers a number of possibilities, including tuning of resistance and activation energy for optimum performance, self-bias correction for fixed pattern noise, and direct addressing. Whilst at an early stage of development, this technology has the potential to significantly impact on the next generation of IR detectors.