Various pH-sensitive polymers have been developed for biomedical applications and recently attracted much attention for pH targeting nanomedicine. Recent development of pH-sensitive polymer systems that sharply respond to specific pH environment in solid tumors and cells and demonstrate modulated properties ($e.g.$, triggered drug release at extratumoral pH, ligands or cell penetrating peptides exposure by shielding/deshielding mechanism or by pop-up mechanism, and endosomal pH targeting) have led significant progress in tumor targeting technology. These pH-sensitive polymer systems combined with nanotechnology could be utilized as an alternative strategy to traditional targeting systems to overcome major problems in current chemotherapy represented by non-specific tissue distribution of the drugs, tumor heterogeneity, and multidrug resistance (MDR) against anticancer drugs. This review highlights recent progress of pH-sensitive polymers with special emphasis on pH targeting nanomedicine.