The discovery of imperfections in an other wise ideally perfect crystal is one of the most fascinating aspects of solid state science. Till now, we have assumed that all lattice structures were perfect or ideal but in real practice, crystal lattices have substantial number of defects. These defects affect the mechanical, electrical and optical properties of the solids. An ideally perfect crystal is one which has the same unit cell and contains the same lattice points throughout the crystal. The term imperfection or defect is generally used to describe any deviation of the ideally perfect crystal from the periodic arrangement of its constituents.