In this paper, some general observations are made concerning the formation of preferred orientation in thin films deposited from the vapour phase, and also in the case where some of the depositing atoms or ions have greater than thermal energies. As a general principle, we postulate that the structures observed represent a minimum of energy for the set of configurations available to the system. The contributions to the energy arise from surfaces and interfaces as well as from the elastic strain energy in the bulk. The implications of this postulate are studied with reference to the growth of BN and related materials including AIN and amorphous carbon.