The specific mechanisms of charged polymer modulation of retrovirus transduction were analyzed by characterizing their effects on virus transport and adsorption. From a standard colloidal perspective two mechanisms, charge shielding and virus aggregation, can potentially account for the experimentally observed changes in adsorption behavior and biophysical parameters due to charged polymers. Experimental testing revealed that both mechanisms could be at work depending on the characteristics of the cationic polymer. All cationic polymers enhanced adsorption and transduction via charge shielding; however, only polymers greater than 15 kDa in size were capable of enhancing these processes via the virus aggregation mechanism, explaining the higher efficiency enhancement of the high molecular weight molecules. The role of anionic polymers was also characterized and they were found to inhibit transduction via sequestration of cationic polymers, thereby preventing charge shielding and virus aggregation. Taken together, these findings suggest the basis for a revised physical model of virus transport that incorporates electrostatic interactions through both virus-cell repulsive and attractive interactions, as well as the aggregation state of the virus.