A new framework for fault diagnosis, called structured hypothesis tests, is proposed. The basic idea is to construct the diagnosis system by combining a set of hypothesis tests. In this way, the task of diagnosis is transferred to the task of validating a set of different models with respect to the measured data. Arbitrary types of faults, including multiple faults, can be handled. That means that one single diagnosis system can diagnose faults of many different types. When using structured hypothesis tests, existing diagnosis methods such as residual generation, parameter estimation, and statistical methods, become parts of one common framework.