There can be little doubt that the morphology of the polymer matrix, the geometrical arrangement of the reinforcing elements and the existance of flaws, all on a mesoscopic scale between a few micrometers to several millimeters, have a strong influence on the performance of composite materials. Mesomechanics describes the properties of these mesostructures. Most of the latter are a result of the particular processing conditions used for the various types of composites. This is demonstrated tin form of examples of the author＇s own experience) for an unfilled semicrystalline polypropylene, a blend of epoxy resin with an elastomeric toughener, an injection moldable short fiber reinforced polyethyleneterephthalate, a discontinuous aligned fiber/polyethersulfone composite, and a high performance continuous fiber reinforced polymer composite system respectively. Mesostructural design is discussed with regard to some possibilities of how to optimize the various mesostructures of these materials, in order to achieve a particular property profile.