Two principal types of polymers are currently used for polymer flooding: synthetic polymers called partially hydrolyzed polyacrylamides (HPAM) and a biologically-produced polysaccharide known as xanthan gum (XG). The generalized structures of these two polymers are shown in Figures 1 and 2, respectively. Although both types of materials have been successfully used in field applications, each suffers limitations that result in process inefficiencies or loss of cost effectiveness. Problems common to both polymer types include difficulties encountered with injection of polymers, especially where the reservoir permeability is low; interactions between surfactants and polymers; degradation caused by the presence of oxygen; and availability of bactericides which are compatible with the polymers. Problems specific to the use of XG include bacterial degradation, injection well impairment, and filtration requirements. Problems encountered primarily with HPAM include viscosity loss in brine, especially brines containing calcium and magnesium ions, and the necessity for special handling to prevent degradation by shearing. Although field tests are being conducted with both types of polymers, the majority of the field projects are using HPAM, probably because of economic considerations.