Computer‐aided variational calculations were made to examine the stability of various lattice defects in the flux line lattice which exist in the mixed state of Type II superconductors. The stable configurations agreed with the direct experimental observations of Essmann and Tra¨uble, if the dislocations and vacancies were assumed to be associated with pinning centers. The presence of pinning centers can also explain the frequencies with which the various defects are observed. The Peierls potential for dislocations was also calculated, as well as the theoretical shear stress for the perfect lattice.