review is made of the main areas of current research interest in modelling and predicting the microstructural/property changes occurring in the heat affected zone in fusion welding. The main subject areas considered are single run welds, multi-run welds, and pulsed welding. The key to modelling in all these cases is the establishment of good equations for the weld thermal cycle. Modelling of other phenomena such as precipitate dissolution, grain growth, phase transformation, property changes, etc. are then tractable. Heat flow equations are well established for single run welds, but there are difficulties in defining the complete thermal cycle in multi-run welding. Pulsed welding, on the other hand, offers good possibilities for accurate modelling and, hence, a better degree of microstructural prediction may be possible in this case. Data are shown for T10 steel