Nanoparticles hold great promise in tumor targeting and molecular imaging because they can co-deliver therapeutic drugs and imaging agents to the tumor site with a single entity. Nanoparticles modified with ligands against moieties overexpressed on tumor tissues have gained increasing attention due to their active targeting mechanisms. Peptides are well suited for nanoparticle targeting modifications because they are small, easy to synthesize and typically non-immunogenic. Herein, we review the peptide-modified nanoparticles used for tumor targeting therapy and molecular imaging based on the classification of peptide-targeting ligands. The development of targeting peptides and nanoparticles will also be discussed.