A discussion is presented on the thermomechanical simulation of the welding process in general and modelling of residual stresses and deformations in particular, in order to better understand what factors affect the accuracy of modelling and thus determine what information needs to be included and what can be ignored in the modelling process. A number of thermomechanical models are classified based on their accuracy. The concept of weld restraint is discussed and related to residual stresses and deformations. The influence of material modelling and simplifications in geometry on residual stress and deformation simulation is also considered for single run and multirun welds.