This paper describes an expert system and its application to structures. Data from vibration measurements are used to assess the condition of structures. Good knowledge of different technical fields (e.g. civil engineering, measuring techniques and system identification) is therefore required. In order to assist the engineer in solving this difficult task, an expert system was developed which is able to help in planning and performing measurements, evaluating the measured values, helping in condition assessment based on the measured data. After stating the objectives of the research project, a general overview of system identification and expert systems is given. The third section describes the expert system program and its knowledge bases in detail. In Section 4 the assessment of structures, which is still under development, is discussed. Finally, as an application of the expert system, the numerical and measuring investigations of a cable-stayed bridge are described.