This chapter describes models of wheat, potato, and maize. It provides measures of performance based on comparisons of simulations with experimental data from research sites and in farm fields. Restrictions in growth due to shortages of water or nitrogen (N) were added to these models via reduction factors based on the ratios of supply to demand (water) or whole-plant concentration to a supposed optimum concentration of N. The mechanism of response to water shortage is that stomata close to conserve water; this restricts CO2 uptake, and growth slows. Phenological development determines the duration of the crop in real time. Because most processes are temperature dependent, calculations are made in thermal time—the time integral of a temperature-response function above a base temperature. Uptake rate is limited to 10% of the water above the lower limit of extraction in any layer in any day.