There is a wide variety of polymeric systems in which relaxation processes play an important role but it would not be practical to attempt to cover them comprehensively in a meaningful way. Two types of systems, semi-crystalline polymers and miscible blends, have been chosen for exposition. The first type of system deals with structural complexity. Semi-crystalline polymers, even if chemically homogeneous, invariably are complex in consisting of two phases. Although the phases are distinct they interact with each other strongly and relaxational behavior reflects this. Molecular modeling has played a role in understanding these interactions. Miscible blends on the other hand are by definition structurally homogeneous. However, they are quite complex in that the constituent molecule types interact with each other and the results can be unexpected. Simulations can play an important role in probing these effects.