Important elements for an efficient tumor targeted delivery are cancer-specific de novo- or over-expression of target receptors and their availability on the tumor cell surface. Peptides can be designed to selectively bind to such receptors and act as tumor-targeting ligands, as has been revealed in several preclinical studies. Notably, the amino acid sequences of these peptides can be chemically modified to prevent enzymatic degradation and improve the stability of the peptide. Furthermore, tumor-targeting peptides can be conjugated to the surface of nanosized drug carriers for the selective delivery of anticancer drugs to tumors. In this review, tumor receptors for which targeting peptides have been identified are described, and their targeting potential is considered. Various chemical modifications of peptides are thoroughly described, and targeting peptides as well as peptide-functionalized nanocarriers are critically discussed. The limitations of active targeting nanocarriers are evaluated in detail, and an outlook on the potential of tumor-targeting peptides and their clinical applications is provided.