The capability to accurately predict the target acquisition performance of thermal imagers is extremely important for several reasons. The predictions can serve as a guide for weapons system development, and they can be used to decide if a particular design will satisfy established requirements. The predictions play an important role in war games and because of this they influence engagement tactics. They also aid in establishment of future weapons system requirements. Target acquisition performance prediction is also an extremely complex task. Several models of thermal imaging system performance have been developed over the years, many of them specialized to only a few military scenarios. The continued improvement in E-O technology combined with the need to predict acquisition performance for a wider range of scenarios have forced an evolution of the models. Systems analysts and model developers have been challenged in several areas related to model development. A brief review of the history of target acquisition model development is followed by an outline of some of the challenges we face at present. Some possible future directions are then discussed.