A major aim of in vivo gene transfer strategies is the efficient and specific delivery of therapeutic genes into the desired target tissues. Non-viral vectors, based on naked plasmid DNA or DNA complexes with cationic lipids or polycationic polymers, are attractive gene delivery vehicles because of their ease of manipulation, stability, low cost, safety, and their high flexibility concerning the size of the delivered transgene. A variety of non-viral vectors effective for gene transfer in cell culture have been developed. However, efficient and target specific in vivo gene delivery remains a major challenge. Compared to cell culture application, in vivo gene delivery faces a variety of additional obstacles including anatomical size constraints, and an environment of interactions with... biological fluids and extracellular matrix. Furthermore, unspecific interactions with non-target cells can be a major obstacle to targeted gene delivery in vivo. Physical parameters of the transfection complexes, such as particle size, charge and stability, are critical factors determining circulation time, biodistribution and transfection efficacy in vivo. Transfection complexes have to be small enough to pass physiological barriers, inert against unspecific interactions with blood components and non-target cells, but allow specific binding to the target cells. After uptake into the target cell, escape from the endosomal compartment and nuclear uptake of the DNA are critical steps for efficient transfection. The present review focuses on the use of various polycation/DNA-based transfection systems for in vivo gene delivery.