The rheological properties of bitumen are improved by addition of small quantities of polymers. A review of different polymers such as Styrene-Butadiene-Styrene (SBS) Styrene-Butadiene-Rubber (SBR), Ethylene Vinyl Acetate (EVA) and Poly Phosphoric Acid (PPA) with emphasis on SBS and its microstructure is presented in this paper. The polymers are divided into two groups: elastomers and plastomers. SBS, an elastomer, has shown greater potential compared to other modifiers when blended with bitumen and improves the mechanical properties and rheological behavior of conventional bitumen. SBS is made up of styrene and butadiene monomer units. It was found that maltene interacts preferentially with the polybutadiene unit of SBS, whereas asphaltene interacts predominantly with the polystyrene unit. SBR is another elastomer which also has polystyrene and polybutadiene units with a different structural make up. EVA is a plastomer which when blended with bitumen has improved the rheological properties and stability, provided the asphalt has asphaltene content below a critical level. PPA is a liquid mineral polymer. Addition of PPA to asphalt contributes to more interactions within the asphaltenes network, thus increasing the elastic behavior. A study was also conducted to determine the influence of asphaltenes and maltenes on the rheological properties of polymer modified binders.