Critical Infrastructures are an essential asset in modern societies and our everyday life is heavily dependent on their reliable and secure operation. The problem of controlling and managing critical infrastructures is becoming more and more difficult as they are increasing in size due to the growing demand for the services they provide and the geographical spread required. As these infrastructures become larger and more complex, fewer people understand how these networks work and the interactions between all the components. Thus, models are necessary so as to accurately predict their behavior under steady state or under failure/attack scenarios. This chapter provides a review on modeling and simulation approaches of critical infrastructures and in particular of electric power, telecommunications, water supply and drainage systems, and transportation systems.