This work deals with the mechanical behaviour and fracture mechanics of parts with chemically, thermally or mechanically treated surfaces, or with inorganic coatings (thin or thick), created in order to achieve special mechanical, chemical, thermal, electrical, magnetic or optical properties. It explains how the strength or fatigue resistance are related to the parameters of surface treatment. The topic is not limited to one class of materials or surface treatment, but is dealt with in such a way that the information may be used for metallic and nonmetallic (e.g. ceramic) components and various kinds of treatment and their combinations, corresponding to modern design trends. Theoretical principles are explained and gives relatively simple formulae and methods (such as those for measuring mechanical and fracture mechanics properties) which can be used directly in research, design and development departments. Practical examples are also given.