This chapter develops the mathematics of discrete and continuous models for a single dynamic quantity. Separate sections introduce each type of model via simple examples and simulations, and two additional sections present the principal graphical methods: cobweb analysis for discrete equations and phase line analysis for continuous equations. The final section uses the graphical methods as motivation for the development of the linearized stability method. The examples and problem sets feature case studies for a variety of biological settings, including fishery models, insect population models, resource management, and lake eutrophication.