The effectiveness of many promising drug candidates (e.g. anticancer agents) awaits the development of drug forms capable of delivery of their drug load specifically to particular sites of an organism or a cell. To make universal and efficient drug carriers, administered drug-loaded vehicles should be able to reach the pathologic zone, recognize and bind their targets at a therapeutic concentration before clearance from the organism. Numerous methods have been developed to couple drug vehicles with active targeting substances - including monoclonal antibodies and substrates or ligands for pathologic cell receptors. Other approaches have included the use of such factors as decreased pH and elevated activity of enzymes in tumor tissues and the hypoxic environment inside the tumor core. This review makes an attempt to analyze the main factors that influence targeting on the kinetic or thermodynamic level that may provide the basis for a strategy to develop and improve drug delivery systems.