In this course the physics behind the operation of modern infrared (IR) detectors and sensors systems will be elucidated. This will include not only the physics and technology behind the detectors themselves but also the associated technologies such as optics, coolers and readout integrated circuits (ROICs). The course will begin with a history of the development of IR detectors. Next basic concepts needed to describe the performance of the detectors and sensor systems will be described and finally there will be examples of applications to modern IR sensor systems used in the ground, airborne and space domains.