This chapter describes the main purposes of computer simulation and gives an overview of the main issues that should be regarded when developing computer simulations. While there are two basic ways of representing a system in a simulation model – the equation-based or macroscopic approach and the individual-based or microscopic approach – this chapter (as the rest of the handbook) focuses on the latter. It discusses the various options a modeller faces when choosing how to represent individuals, their interactions and their environment in a simulation model.