Models are particularly important in the study of complex systems. The role of the computer is particularly relevant: on the one hand, it allows one to simulate phenomena which would otherwise be impossible to deal with, but on the other hand it is also a laboratory for the study of complexity itself. One of the most striking experiences is that of a programmer, who has set up a simple program like e.g. a cellular automaton, who observes its behaviour with surprise. Indeed, there is in general no obvious rule to induce the overall system behaviour from the knowledge of the rules of the interaction of its elements – and this is particularly surprising in a completely artificial system like a computer program, where we know with absolute precision what the microscopic rules are!Among the many tools which are available some seem particularly interesting, for their conceptual importance, their practical applications and, last but not least, their capability to be understood and to impress their user: cellular automata, genetic algorithms, genetic networks, agent-based models.