Complex systems are everywhere among us: telecommunication networks, computers, transporting vehicles, and electrical appliances are well known examples. Designing reliable systems and determining their availability are both very important tasks for managers and engineers, since reliability and availability have a strong relationship to other concepts such as quality and safety. Furthermore, these tasks are extremely difficult, due to the fact that analytical methods can become too complicated, inefficient or even inappropriate when dealing with sophisticated systems. In this paper we present the basic ideas behind a simulationbased method, called SAEDES, which can be very useful when determining availability for a wide range of complex systems. The method is implemented in C/C++ using two different algorithms, SAEDES_A1 (component-oriented) and SAEDES_A2 (system-oriented). Two case-studies are introduced and analyzed using both algorithms, which allows us to compare the associated results.