Composite micromechanics deals with the relationships between macroscopic properties of heterogeneous polymer matrix materials such as lamina or particulate composite and its microstructural parameters. In composite mechanics, this is the largest length scale at which structural details are considered. The primary objective of this article is to ascertain the state of the art regarding the understanding and prediction of the properties of polymer composites reinforced with microsized fibrous or particulate inclusions as affected by the inclusion content, shape, size, properties, orientation, and quality of the interface/interphase phenomena.