The IR engineer has the choice of many types of cryogenic cooling systems which can be used for cooling components of IR systems. This article is intended to give some guidance in this selection process to the IR engineer, who may not be intimately familiar with various cooler types. The article briefly describes the characteristics of cryogenic coolers which are suitable for cooling components in IR systems to temperatures in the range of 4-120 K. It covers systems which are presently available, or will be available in the foreseeable future. The principles of operation of commonly available cooler types are described, the characteristics of each type are discussed, and the probable areas of application for each type are indicated. The following cooler types are discussed: open cycle systems using expendables (liquid cryogen, solid cryogen, and Joule-Thomson coolers); continuous flow, closed cycle refrigerators (Joule-Thomson and reversed Brayton cycle refrigerators); intermittent flow, closed cycle refrigerators (Stirling cycle; Gifford-McMahon cycle, and Vuilleumier cycle refrigerators, plus split cycle refrigerators based on these three cycles); and passive radiators. A bibliography, containing references to more detailed information on all cooler types, is included.