The microstructure developed during weld solidification and the effect of the various welding and materials parameters is discussed. It is shown that weld solidification is in several ways fundamentally different from solidification in conventional casting in that it is a dynamic (moving heat source) process in which the `mould' material (the base metal) has approximately the same composition as that of the solidifying metal. The moving arc also has the effect of continuously varying the solidification conditions, e.g. the temperature gradient and degree of convection, in the melt. The additional modifications to microstructure during cooling of the weld metal are also briefly discussed