It is shown that direct observations by electron microscopy of the interaction of lattice dislocations with high angle grain boundaries can give valuable information about not only properties but also interfacial structure. The experimental results suggest that during low temperature yielding boundaries act primarily as sources of dislocations rather than as barriers to slip. At high temperatures, the incorporation of lattice dislocations by the rearrangement of boundary structure is important in boundary sliding and recrystallization. Except in a relatively few special cases, the simple boundary model of a dislocation network embedded in a Coincident Site Lattice interface appears to be inadequate for the understanding of these interactions