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The Environmental Policy Integrated Climate (EPIC) and Agricultural Policy/Environmental eXtender (APEX) models have been developed to assess a wide variety of agricultural water resource, water quality, and other environmental problems. The EPIC model is designed to be applied at the field scale. APEX is a direct extension of EPIC that can also be applied to fields as well as to more complex multi-subarea landscapes, whole farms, and watersheds. This article describes key model components of EPIC and APEX, including different options for simulating surface runoff, evapotranspiration, soil erosion, and other processes. Field-scale calibration and validation procedures are then described for both models, with an emphasis on important calibration parameters and guidance regarding logical sequences of calibration steps. Additional calibration and validation guidance is further provided for applications of APEX at the landscape and watershed scales. Two calibration and validation case studies are presented: one for an EPIC plot study and one for an APEX study of a 35 ha field in north-central Missouri. Research and development needs for both models are also discussed.