  Martensites are metastable phases, possessing a characteristic morphology,
usually formed during a fast quench accross a structural transition. We attempt
to understand these morphological features using a coarsegrained free energy
functional ${\cal F}[\epsilon ; \Phi]$ which contains, in addition to the usual
strain fields $\epsilon_{ij}$ (the `` order parameter'' for the transition),
the ``vacancy'' field $\phi$ which arises due to the geometric mismatch at a
parent-product interface. The relaxation of this mismatch is slow compared to
typical front propagation times and hence $\phi$ is essentially frozen in the
reference frame of the growing martensite front. Minimisation of ${\cal F}$
then automatically yeilds typical martensite morphologies. We demonstrate this
in two dimensions for the square to rhombus transformation and obtain
internally twinned martensites, which grow as thin strips, for ``hard''
martensites (e.g., Fe-based alloys) or with a `single-interface', for ``soft''
martensites (e.g., In-Tl alloys).
