Although monoclonal antibodies have demonstrated clinical potentials as tumor targeting agents, poor tumor penetration of the antibodies due to the size of molecules and liver/bone marrow toxicity by non-specific uptake of the antibodies are the two major limitations of antibody therapy. Peptidic targeting agents may ease the problems associated with antibody cancer therapy. Combinatorial libraries displayed on microorganisms have successfully been utilized to discover cell surface-binding peptides, which can be tumor-targeting agents. Among many molecular display techniques, phage display has been the most popular approach. Peptides can be used as targeting molecules of receptor-targeted toxins and gene therapy, imaging and/or therapeutic agents, and nano-medical technologies. Recent results from preclinical studies with various peptides support their targeting potential and suggest that the role of peptides as targeting molecules in drug development should be further exploited.