Cationic liposome/micelle-based systems have come closer to providing clinically effective gene delivery than any other chemical nonviral delivery systems to date. These systems are formed from either a single synthetic cationic amphiphile (known as a cytofectin; cyto for cell and fectin for transfection [i.e., gene delivery and expression]) or, more commonly, from the combination of a cytofectin and a neutral lipid such as dioleoyl l-α-phosphatidylethanolamine (DOPE) or cholesterol (Chol) (see Fig. 1). There are at least 40 cationic liposome/micelle systems that have been reported to mediate nucleic acid delivery to cells, of which a number have been commercialized (see Table 1) (1–25). More are being reported all the time, but in each case, the key ingredient is the cytofectin used. The structures of a number of representative diverse cytofectins are shown (see Fig. 1). Although hydrophobic regions are reasonably similar, polar linkers and cationic head groups vary quite substantially. Typically, cytofectin and neutral lipid components are mixed together in an appropriate mole ratio and then induced or formulated into unilammellar vesicles by any one of a number of methods, including reverse-phase evaporation (REV) and dehydration-rehydration (DRV) (1,2) (see Table 1). Alternatively, cytofectins may be assembled into micellar structures after being dispersed in water or aqueous organic solvents (1,2) (see Table 1). Unilammellar vesicles or micelles may then be combined with nucleic acids to form cationic liposome/micelle-nucleic acid complex (lipoplex; LD) mixtures consisting of nanometric complex structures that are able to deliver nucleic acids into cells (see Fig. 2 on p. 108–111).