Modifiers have been used to improve the performance of asphalt/aggregate mixtures for building roads for over the past hundred years and during that time the industry has evolved into a sophisticated sector that is heavily regulated by national and European standards. The modifiers used take many different forms: polymers, latex, and many chemical additives. Determining how modifiers actually affect the performance of a mixture and establishing construction quality control procedures for these modifiers can be extremely difficult. In the past these difficulties have caused many specifying agencies to avoid the use of mixture modifiers when they could be of benefit to the performance of the road. This review explores the type of polymers used in asphalt, why they are used, where they are used in terms of applications and the benefits they offer to industry and the road user. In particular, the reader will understand how polymers can be used to enhance the functionality of asphalt, that is to overcome deterioration mechanisms by enhancing asphalt stiffness or flexibility, or by making it more resistant to deformation (rutting) caused by traffic. This review of polymers in asphalt is supported by an indexed section containing several hundred key references and abstracts selected from the Polymer Library. It is aimed at anyone who has an interest in polymers and their highway applications.