The developmental rate of insects is an important parameter in studies of insect population dynamics as well as in biological and chemical control of agricultural pests. Traditionally, models used to predict developmental rates are empirical functions of temperature. Such models are limited from a theoretical perspective because they are not explanatory and fail to include the known effects of variables other than temperature in determining developmental rate. A new approach is developed here that is based on the assumption that each species has a characteristic size (mass) at the completion of its larval or nymphal stage and that the rate of growth to this ultimate size determines the rate of development. Growth rate is modeled as a function of consumption rate and respiration (influenced by temperature) as well as approximate digestibility (influenced by diet quality). An analytic solution of this growth rate model gives the developmental rate as a function of temperature and diet quality. This approach was applied to a grasshopper,Melanoplus sanguinipes. Reasonable parameter choices generated a realistic developmental rate curve. Advantages of this approach over previous models include (1) the model is mechanistic and explanatory, (2) development is a function of nutrition as well as temperature, (3) the parameters have clear ecological meanings and are, for the most part, directly measurable, and (4) variable weight at the end of the life stage can be included in the model.