1.1 ,This guide presents a systematic approach to the classification and description of computer codes used in groundwater modeling. Due to the complex nature of fluid flow and biotic and chemical transport in the subsurface, many different types of groundwater modeling codes exist, each having specific capabilities and limitations. Determining the most appropriate code for a particular application requires a thorough analysis of the problem at hand and the required and available resources, as well as a detailed description of the functionality of potentially applicable codes.