Self-assembling complexes from nucleic acids and synthetic polymers are evaluated for plasmid and oligonucleotide (oligo) delivery. Polycations having linear, branched, dendritic. block- or graft copolymer architectures are used in these studies. All these molecules bind to nucleic acids due to formation of cooperative systems of salt bonds between the cationic groups of the polycation and phosphate groups of the DNA. To improve solubility of the DNA/polycation complexes, cationic block and graft copolymers containing segments from polycations and non-ionic soluble polymers, for example, poly(ethylene oxide) (PEO) were developed. Binding of these copolymers with short DNA chains, such as oligos, results in formation of species containing hydrophobic sites from neutralized DNA polycation complex and hydrophilic sites from PEO. These species spontaneously associate into polyion complex micelles with a hydrophobic core from neutralized polyions and a hydrophilic shell from PEO. Such complexes are very small (10-40 nm) and stable in solution despite complete neutralization of charge. They reveal significant activity with oligos in vitro and in vivo. Binding of cationic copolymers to plasmid DNA forms larger (70-200 nm) complexes. which are practically inactive in cell transfection studies. It is likely that PEO prevents binding of these complexes with the cell membranes ("stealth effect"). However attaching specific ligands to the PEO-corona can produce complexes, which are both stable in solution and bind to target cells. The most efficient complexes were obtained when PEO in the cationic copolymer was replaced with membrane-active PEO-b-poly(propylene oxide)-b-PEO molecules (Pluronic 123). Such complexes exhibited elevated levels of transgene expression in liver following systemic administration in mice. To increase stability of the complexes, NanoGel carriers were developed that represent small hydrogel particles synthesized by cross-linking of PEI with double end activated PEO using an emulsification/solvent evaporation technique. Oligos are immobilized by mixing with NanoGel suspension, which results in the formation of small particles (80 nm). Oligos incorporated in NanoGel are able to reach targets within the cell and suppress gene expression in a sequence-specific fashion. Further. loaded NanoGel particles cross-polarized monolayers of intestinal cells (Caco-2) suggesting potential usefulness of these systems for oral administration of oligos. In conclusion the approaches using polycations for gene delivery for the design of gene transfer complexes that exhibit a very broad range of physicochemical and biological properties, which is essential for design of a new generation of more effective non-viral gene delivery systems.