This study presents a calculation of the free energy of the ice?water interface using molecular simulation. The method used is an adaptation of the cleaving method, introduced by Broughton and Gilmer, and subsequently enhanced by Davidchack and Laird. The calculation is direct in the sense that an interface is formed during the simulation: isolated ice and water systems are transformed, via a sequence of reversible steps, into a single system of ice and water in contact. The method is essentially computational, that is, it does not correspond to any possible physical experiment, since non-physical potential energies are introduced (and subsequently removed) during the transformation process. The adaptation of the method to water presented significant challenges, notably the avoidance of hysteresis during the transformation, and the devising of an?external? energy potential to control the position and orientation of water molecules. The results represent the first direct calculation by simulation of the solid?liquid interfacial free energy for a model of a molecular (as opposed to atomic) system.