Adequate representation of the water cycle in cropping systems remains as one of the major concerns for crop model developers as it is involved in many critical processes such as the determination of the energy balance and crop temperature, the determination of crop productivity, and the movement of nutrients in the soil and their uptake. Even though the water budget equation is rather simple, the scarcity of measurements and data to specify the parameters and evaluate the component processes, as well as the myriad of empirical and mechanistic approaches available, makes it difficult for crop modelers to decide on suitable approaches to incorporate in their models. This chapter presents a summary of the most common approaches that are included in crop simulation models, the parameterization of the equations involved, and the main limitations currently observed.