The Thesis has developed a methodology for evaluating the reliability level of buildings in use. The overall reliability of the building is characterized by the global reliability index which allows to quantify and compare the reliability levels of different buildings and, depending on the type of use of the building, determine the lowest permissible level. New methods have been developed in the work – the method of quantitative risk classification of buildings and the method for determining the reliability level of individual structural elements in operation. A large-scale study on the state of existing public buildings in Latvia has also been conducted and the practice of building inspection during operation in Europe has been analysed.