Infrared detection systems include the systems for detecting point targets and the thermal imaging systems for detecting extended source targets. As a system-level performance description parameter, the operating range can be used to characterize the quality of a detection process. The equation of the operating range for a detector depends on whether the source is a point one or an extended one. Besides, it also depends on whether the desired energy is a radiation energy form or a photon form. The operating range equations under different conditions are presented. The influence of system parameters, such as the F/# and diameter of optics and the field angle of a detector, on the operating range of both a search system and a thermal imaging system is discussed. Finally, the understanding and comprehension for the operating range of infrared detection systems are given.