I will review some aspects of the theory of the Wulff shape in the
context of the Ising model. According to the Wulff construction the
shape of the equilibrium crystal is determined by the surface tension
considered as a function of the interface orientation. We present some
(conjectured) approximate solutions and some rigorous results
concerning this function, in the case of a lattice gas, and apply them
to study the shape of the equilibrium crystal and, in particular, the
shape of the facets of this crystal.