If we would like to do a quantitative analysis of a complex system, we can build a mathematical model of the system. If this model is tractable, we can use it to analyse the system "analytically", or we can build a computer simulation according to the model. However, if the system is made up of independent agents, where the agents interact with each other and/or with their environment, we can build a computer simulation representing each agents and their interactions. This class of simulations is called agent-based simulations. In this seminar lecture, we will discuss the fundamentals of agents-based simulations: definitions, when to use, advantages, drawbacks, examples.