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Inrecent years, polymeric nanoparticles have appeared as a mostviable and versatile delivery system for targeted cancer therapy.Various in vivo studies have demonstrated that virus-sized stealthparticles are able to circulate for a prolonged time and preferentiallyaccumulate in the tumor site via the enhanced permeability and retention(EPR) effect (so-called “passive tumor-targeting”).The surface decoration of stealth nanoparticles by a specific tumor-homingligand, such as antibody, antibody fragment, peptide, aptamer, polysaccharide,saccharide, folic acid, and so on, might further lead to increasedretention and accumulation of nanoparticles in the tumor vasculatureas well as selective and efficient internalization by target tumorcells (termed as “active tumor-targeting”). Notably,these active targeting nanoparticulate drug formulations have shownimproved, though to varying degrees, therapeutic performances in differenttumor models as compared to their passive targeting counterparts.In addition to type of ligands, several other factors such as in vivostability of nanoparticles, particle shape and size, and ligand densityalso play an important role in targeted cancer chemotherapy. In thisreview, concept and recent development of polymeric nanoparticlesconjugated with specific targeting ligands, ranging from proteins(e.g., antibodies, antibody fragments, growth factors, and transferrin),peptides (e.g., cyclic RGD, octreotide, AP peptide, and tLyp-1 peptide),aptamers (e.g., A10 and AS1411), polysaccharides (e.g., hyaluronicacid), to small biomolecules (e.g., folic acid, galactose, bisphosphonates,and biotin), for active tumor-targeting drug delivery in vitro andin vivo are highlighted and discussed. With promise to maximize therapeuticefficacy while minimizing systemic side effects, ligand-mediated activetumor-targeting treatment modality has become an emerging and indispensableplatform for safe and efficient cancer therapy. [ABSTRACT FROM AUTHOR]