This volume of the encyclopedia is devoted to the basics of modern theory and practice of reliability. It presents mathematical models and methods for predicting the reliability of machines, shows the prospects for the development of reliability theory. A separate section of the publication is devoted to the factors that determine reliability, methods for determining and improving reliability indicators are presented, many working examples of the effective use of complex diagnostic methods to improve the quality of machine performance are given. The most complex and most urgent problems are considered today: studies of the reliability of products at the stages of design and operation of technical systems, bench tests of prototypes of machines are described The experience of researching and ensuring the reliability of various complex technical systems: nuclear power plants, rocket and space equipment, aircraft, radio-electronic equipment, road transport, pipelines for oil and gas transportation .