The development of theoretical models to simulate the microstructures of weld metals from classical nucleation and growth theory still provides major challenges. Such models are important since they may allow predictions to be made for a wide range of low and high alloy weld metal compositions. In this paper, the nature of the acicular ferrite microstructure associated with good strength and toughness in weld metals (low alloy compositions) is examined from both a historical and contemporary perspective. Attempts are made to rationalise the different views on acicular ferrite development. A distinction is made between acicular ferrite microstructures where there are multiple impingements between different reaction products nucleated and growing from intragranular inclusions, and microstructures in more highly alloyed weld metals. These may include low temperature transformation products growing from prior austenite grain boundaries and ferrite plates growing relatively unimpeded from intragranular inclusions. From the literature, a suitable methodology for physically modelling weld metal microstructure development at prior austenite grain boundary and intragranular sites appears to be available. This applies the Johnson-Mehl-Avrami theory to overall reaction kinetics to deal with the simultaneous transformation of two or more phases although only approximating the issue of physical impingement. Some preliminary results from modelling of intragranular ferrite formation are quoted from the literature. The need for appropriate quantitative data for model calibration purposes is paramount and areas for further work are highlighted. A new scheme is briefly described for classifying and quantifying complex microstructures in steels. This provides a means of rationalising the transformation products associated with acicular ferrite and which are the outputs of theoretical models.