This book reviews the behaviour of metals and alloys during welding. In the first part the heat flow in arc welding processes is discussed. The weld thermal cycle is explained in terms of heat input, and the geometry of weld and thicknesses to be welded. The real welding cycle is described in terms of thermal and strain cycles. The weld metal is characterized in terms of fusion stage, absorption of gases and stage of metal crystallization and structural transformation. The metallurgical background of cracking is described by a full set of crackability tests along with the evaluation of metals from the point of view of crackability. Post welding heat treatment is reviewed, and includes the relaxation of stresses induced by welding. Guidelines are given for the selection of steels for welded structures. Several chapters examine the weldability of particular steels, including high strength steels, stainless steels, high alloyed steels, cryogenic steels and other metals and alloys. The theories are quantified in the form of calculations or computing programmes. Readers will find sufficent data for software processing. The use of computing programmes for the evaluation of weldability and optimalization of welding parameters forms a basis for computer-aided technology or computer-aided manufacture. A theoretical background is also provided for the software of robotic technologies which are considered to have a very promising future in this field.