In the world where on-demand and trustworthy service delivery is one of the main preconditions for successful business, service and business process availability is of the paramount importance and cannot be compromised. For that reason service availability is coming into central focus of the IT operations and management research and practice. Still, our understanding of service and process availability is mostly empirical and at best, sketchy. Services are assessed using a mixture of qualitative, quantitative, and analytical methods, with results of varying quality. We introduce a systematic model-based methodology and a tool for service and business process availability assessment. The main advantage of the proposed method is the ability to automatically generate availability models, based on the service/process description and technical infrastructure it is executing on. Therefore, service level agreements can be tested/simulated or return on investment calculation can be performed, without the need for costly experiments and/or actual investments.