The textbook was prepared on the basis of a course of lectures given by the author of the manual for several years to students of mechanical specialties of universities. The manual includes general issues of diagnostics and non-destructive testing, concepts about defects and their types, issues of building and studying models of objects of control and diagnostics, models leading to process failure, diagnostic signs, choosing diagnostic methods, etc. The appendix contains topics of laboratory work, practical exercises, tasks for them and examples of solving some problems.