We have isolated several organ- and tumor-homing peptides by using in vivo phage display. This technology involves the screening of peptide libraries in a living animal. The peptides that result from such a selection home to specific organs or tissues because they recognize molecular ‘addresses’, receptors that are differentially expressed in vascular beds. Targeted delivery of chemotherapeutics, pro-apoptotic peptides and cytokines to tumors using these peptides improved therapeutic efficacy in animal models. Translation of this technology into clinical applications will form the basis for targeting therapeutic and imaging agents in the context of cancer and other diseases.