The breadth of activity in the field of infrared zoom lenses is international in scope. Representative infrared zoom lens systems of the 1990s from the continents of North America, Europe, and Asia and the Middle East are presented. Design considerations unique to the infrared spectrum such as athermalization are given special attention. The various approaches to solving these design problems are described along with the operating characteristics of these systems. Applications include forward-looking infrared scanning systems and target simulators. Tolerance considerations during manufacture are presented. Innovative reflective zoom systems are also considered. Development trends for the next several years are discussed, including the use of diffractive optical elements in infrared zoom lens systems.