Modified binders—obtained by adding thermoplastic macromolecular systems of plastomers or elastomers to traditional pure asphalt cements for road construction—have existed for more than 20 years. These products are still poorly understood scientifically. Significant research must be undertaken to better understand binder relationships between composition, structure, and properties, and the relationships between hot mix performance (rutting, fatigue, thermal cracking) and the characteristics of the modified binders as measured in the laboratory. The current state of knowledge regarding these issues is presented along with a discussion of the basic principles of bitumen-polymer compatibility, the mechanism for modification, and the effect of the components on the properties of the modified binders. Several specific problems related to the use of these products are presented.