Welding, as a modern and highly efficient technology, is presently applied in practically all industrial sectors. An increase in the number of welding applications entails higher requirements related to the quality of welded joints. Numerical analyses enable welding process simulations whose results are very close to reality. The results of such analyses include distributions of temperature fields, metallurgical phases, hardness, plastic strains and stresses. The analysis of these quantities makes it possible to relatively optimise welding technologies and to obtain information about the behaviour of welded structures during the entire production process. Numerical simulations constitute very useful tools supporting production preparation processes and enabling the obtainment of high quality products. Presently, the most complex numerical simulation software programmes make up the package offered by the ESI Group including SYSWELD, PAM ASSEMBLY and WELD PLANNER. All these programmes utilise FEM-based calculations. This article is the first part of a cycle concerning the possibility of utilising numerical techniques in supporting the design of technologies and structures using computer-aided simulations of welding processes and heat treatment.