Abstract: The present invention provides nanostructures that are particularly well suited for delivery of bioactive agents to organs, tissues, and cells of interest in vivo, and for diagnostic purposes. In exemplary embodiments, the nanostructures are complexes of DNA strands having fully defined nucleotide sequences that hybridize to each other in such a way as to provide a pre-designed three dimensional structure with binding sites for targeting molecules and bioactive agents. The nanostructures are of a pre-designed finite length and have a pre-defined three dimensional structure.