The deformation of grain boundaries by crystal lattice dislocations is discussed in detail for the case of symmetric boundaries in simple cubic structures. In particular, It is shown that the motion of grain boundary dislocations that are generated as a result of various combinations of crystal lattice dislocations gives rise to either grain boundary sliding or grain boundary migration or both. The structure and motion of various types of ledges such as glide ledges, climb ledges, and compensated and uncompensated ledges are considered in this analysis. Although the analysis is done for simple cubic structures, the concepts developed are quite general and can be applied to more complex crystal structures. It is also shown that some of the grain boundary dislocation arrays can be suitably described as disclination dipoles, and such a dualistic representation is used to interpret the shape changes associated with the grain boundary deformation.