Two dimensional, non-equilibrium molecular dynamics simulations have been performed to examine the microstructures of both homoepitaxial and heteroepitaxial thin films grown on single crystal substrates. The principal microstructural features to develop within these films are small voids and edge dislocations. Voids form near the surface of the growing film as surface depressions between microcolumns pinch off to become closed volumes. These voids often form in such a way as to introduce dislocations into the crystal with their cores positioned within the voids. Dislocations are also formed during heteroepitaxy at the interface between the substrate and film. These dislocations tend to be mobile. When voids are present in the film and when the lattice misfit is low, dislocations tend to be trapped in the voids or pulled toward them due to dislocation image interactions. Once attached to voids, dislocations are effectively pinned there. When voids are absent or when the misfit is high, dislocations are restricted to the film-substrate interface. In the case of heteroepitaxy, dislocations are found to relieve either tensile or compressive misfit stresses. Misfit stresses may also be accommodated, to some extent, merely by the free volume of the voids themselves.