Simulation modeling is a family of powerful methodologies, which received a strong impulse by the growth of computer power and languages. Knownetlab researchers developed a remarkable expertise in building and studying agent-based models, which are rapidly assuming a great relevance in many social sciences as well as in biology and medicine. Its advantages are really enormous, because they are the laboratory where scientists can test and develop theories, and attempt predictions and scenarios. Moreover, virtual experiments can explore situations not yet evidenced by field or pure speculative research. Finally, the formalization forced by modeling helps inter-theoretical comparisons by making explicit hypotheses, methods and the operationalization of variables, and by allowing the replication of experiments