Individuals differ in many characteristics such as sex and age, which implies that they usually have different chances of reproducing and surviving. Even stronger differences are usually observed if individuals belong to different species. This chapter presents two types of model for the consideration of individual variability. The first is equation-based models that differ from the models of Chapter 4 by the fact that several equations (one for each age class or each species, etc.) are needed. If the dimensions along which individuals differ are too numerous, or if additional features such as adaptive behaviour are included, individual-based models are more appropriate. An example demonstrates that equation-based models and individual-based models, when considering the same features of the modeled population, lead to the same results. It is argued that individual-based models are generally more complex than equation-based models but also more flexible and able to consider more details.