The polycation polyethylenimine (PEI) has recently been widely employed for the design of DNA delivery vehicles. Gene delivery using PEI involves condensation of DNA into compact particles, uptake into the cells, release from the endosomal compartment into the cytoplasm, and uptake of the DNA into the nucleus. Particularly for in vivo gene delivery, optimal coordination and timing between DNA complexation for protection of the DNA from nucleases and the disassembly of the complexes is essential. For in vivo application, DNA complexes have to pass a variety of anatomical and physiological barriers, and an environment of biological fluids and extracellular matrix before reaching their targets. Furthermore, targeted gene delivery is seriously hampered by non-specific interactions with non-target cells. Strategies have been developed to protect transfection complexes from non-specific interactions and to increase target specificity and gene expression.