This chapter considers models where processes like foraging, metabolism, and mortality are continuous, whereas reproduction is assumed to take place as a discrete event at the start of the growth season. The assumption of discrete reproduction is relevant for many organisms living in seasonal environments (winter/summer, dry/wet seasons). The use of several time scales means that the analysis of the dynamics in this chapter will be restricted to simulations using the Escalator Boxcar Train (EBT) framework. The EBT is specifically designed to handle the numerical integration of the equations that occur in physiologically structured models using ordinary differential equations and is particularly well suited for systems with discrete reproduction.