The invention relates to a method for testing the reliability of complex systems, comprising the following steps: establishing a system hierarchy of several levels by identifying components of the system; identifying and assigning failure modes to components; determining test procedures for failure modes; reliable information about the availability of technical systems with the least amount of tests can be obtained by the following steps: identifying a level for allocating availability targets to components establishing a hierarchical decomposition of the system; allocating availability targets to the components identified in the previous step by qualitative methods; calculating a projected availability of all components; identifying components for which availability targets are not met by said projected availability and selecting another test or increasing severity of tests already selected till availability targets are met; and calculating an overall availability of the system.