This paper proposes a concept for an availability modeling tool called OpenSESAME (Simple but Extensive Structured Availability Modeling Environment) . The tool accepts high level graphical descriptions of a given system and automatically converts them into Generalized Stochastic Petri Nets which in turn can then be quantitatively evaluated. The input diagrams allow a hierarchical view of the system and can take into account all aspects which may influence the over-all system's availability. These aspectes include a wide variety of inter-component dependencies. Keywords Fault Tolerance, Availability Modeling, GSPN. 1 Introduction When designing High Availability Systems, availability models can be used to quantitatively evaluate the system's availability, locate availability bottlenecks or superfluous redundancy and thus find an ideal tradeoff between dependability and costs. To obtain a realistic availability model of a complex system, intercomponent dependencies have to be..