To analyze, quantify, and compare the performance of infrared (IR) imaging devices, such as various types of focal plane arrays (FPAs), it is essential to have an in-depth understanding of their figures of merit. In this entry, we discuss a series of figures of merit that are applicable to both cooled and uncooled IR detectors and systems. We provide a more detailed analysis of these figures of merit for the case of uncooled thermal detectors based on micro-electro-mechanical systems (MEMS). In particular, we discuss and analyze the different sources (and mechanisms) of noise present in MEMS IR detectors. These noises inevitably influence the respective figures of merit for the MEMS IR detectors and ultimately affect the fundamental limits of their performance.