Conservation practices are often implemented based on experience and field conditions. More informed conservation planning using simulation models allows targeting placement of practices to critical areas. Simulation models can be used to simulate the effectiveness of various conservation practices over long time periods and over large areas, thereby providing feedback to future conservation planning. This eliminates ineffective conservation practices and expands the temporal and spatial scales of conservation measures. The Agricultural Policy/Environmental eXtender (APEX) model has been developed to assess a wide variety of agricultural water resource, water quality, and other environmental problems. The major APEX model components and available APEX geographic information system (GIS)–based interfaces that support APEX applications are described in this chapter. Case studies that describe and discuss the model use for conservation planning are provided to further explore the potential of this simulation model to enhance conservation planning.