Each normal organ and pathological condition appear to contain organ- or disease-specific molecular tags on its vasculature, which constitute a vascular "zip code" system. In vivo phage display has been exploited to profile this vascular heterogeneity and a number of peptides that home specifically to various normal organs or pathological conditions have been identified. These peptides have been used for targeted delivery of oligonucleotides, drugs, imaging agents, inorganic nanoparticles, liposomes, and viruses. Identification of the receptor molecules for the homing peptides has revealed novel biomarkers for target organs. In tumors many of these receptors seem to play a functional role in tumor angiogenesis. Recently, tumor homing peptides have entered clinical trials. Results from several Phase I and II trials have been reported, and a number of trials are currently ongoing or recruiting patients. In these trials no dose-limiting toxicity has occurred and all combinations of peptide-targeted therapies have been well tolerated.