Bitumen is a by-product of the fractional distillation of crude oil, but is also found in natural deposits. It has a unique combination of excellent water-proofing and adhesive properties which have been used effectively for more than 5000 years. Bitumen is a low-cost thermoplastic material which is widely used in roofing, road and pavement applications. However, it is brittle in cold environments and softens readily in warm environments. One of the many methods employed to toughen bitumen is to blend it with polymers, either virgin or scrap, to produce polymer modified bitumen (PmB). In this chapter, an introduction to bitumen structure and properties, and a short review of the published literature on PmB is provided.