Brittleness is a significant property considered in product design and the research and development of materials. However, for a long time the methods to determine brittleness have been largely "hand-waving" arguments or else circumferential properties-in other words describing numerous properties related to brittleness but not actually quantifying brittleness itself. We have defined brittleness of polymeric materials quantitatively with applications to multiple areas. Relationships between brittleness and both tribology and mechanics have been discovered and are described. Moreover, the definition has been applied in the development of multilayer composite materials; structural integrity of the composites decreases with increasing brittleness. Other applications and the fact that toughness is not an inverse of brittleness are also discussed.