At first a general introduction to the modelling of martensitic phase transformations is given. We then focus on the variational approach most notably developed by Ball and James. We will show how this framework can be applied to areas that were previously thought to be outside its scope. Based on the assumption of energy minimisation, we present a new theory for the formation of (5 5 7) habit plane normals observed in low carbon steels. Compared to existing theories, the most important advantage is that no additional input parameters need to be assumed. Joint work with K. Koumatos.