Phosphorus dendrimers are promising new tools for the delivery of drugs and genes to targets such as tumors. This chapter summarizes advances in the ability of phosphorus dendrimers to deliver genes into nuclei of cells and small interfering RNAs into the cytoplasm as a molecular basis for gene therapy in cancer. There are three different directions in gene therapy: the construction of human genome to create meta-homo sapiens, the delivery of therapeutic genes into cell nuclei to have these genes permanently expressing themselves, and the silencing of unwanted genes. The mechanisms of interaction between phosphorus dendrimers and nucleic acids, and the gene transfer mediated by these dendrimers, are practically the same as for poly(amidoamine) dendrimers. The inter-dendrimer distance is independent of the DNA length in complexes with a protonated dendrimer. This phenomenon was explained by the electrostatic complexation of a negatively charged DNA strand with the positively charged protonated dendrimer.