Block copolymers composed of a cationic segment and a hydrophilic segment spontaneously associate with polyanionic DNA to form block copolymer micelles. The distinct feature of the associate is that the core of the polyion complex between DNA and the polycation is coated by a layer of the hydrophilic polymer. The characteristic core-shell structure endows the associate with a high colloidal stability and reduced interaction with blood components. These desirable properties are the major advantages of the micellar DNA delivery system for in vivo application. In this article, the synthesis of block copolymers as well as graft copolymers utilized as DNA delivery systems are described. Particular emphasis is devoted to the association behavior and the physicochemical properties of polyion complex micelles entrapping DNA and related substances in relation to the biological aspects of the associates. Biodistribution and the factors that affect the intracellular fate of the micelles is also addressed based on recent studies in this field.