Compositions and methods are provided that are useful for the delivery of therapeutic agents, including nucleic acids. The compositions comprises a non-covalent association complex of. a) a positively-charged backbone and b) at least two members selected from: I) a first negatively-charged backbone having a plurality of attached imaging molecules； ü) a second negatively-charged backbone having a plurality of attached targetting agents； üi) at least one member selected from RNA, DNA, ribozymes, modified oligonucleotides and cDNA endcoding a selected transgene； iv) DNA encoding at lest one persistence factor； and v) a third negatively-charged backbone having a plurality of attached biological agents； wherein the association complex carries a net positive charge and at least one of the two members from group b) is selected from I) iii) or v).
