Giant DNA molecules of size greater than several kbp adopt an elongated coil conformation in aqueous solution. On addition of various kinds of condensing agents, the giant DNA molecules fold into a compact state. With multivalent cations, individual DNA molecules show large discrete transitions between the elongated and compact states. Similarly, with the addition of polymers such as PEG, DNA molecules also switch conformation in an all-or-none manner. In contrast, DNAs fold into a loose compact state with cationic surfactants. The experimental conditions necessary to generate stable soluble particles from single DNA molecules and also to obtain multiple DNA molecular assemblies are outlined. The relationship between the higher-order structure and the efficiency of gene transfection is discussed.