Reaction injection molded polyurethane chemistry has evolved to provide improved properties and processing while meeting competitive cost pressures. The concept of making parts thinner to reduce mass, thus costs, has been introduced previously and has been commercially reduced to practice. Now, the incorporation of light weight microspheres into RIM parts lowers specific gravity equal to that of TPO, about 0.92 gm/cc. Meeting performance requirements with thinner and lighter RIM materials while retaining the superior paintability and design flexibility of RIM has now been demonstrated. New understanding of polymer performance and continued development of filler technology make these unique and effective composites possible. Properties, processing, and field experience with these systems will be discussed.