Polymeric micelles from pH-sensitive block copolymers have been designed for targeting tumor acidity or endosomal pH in tumor cells. The micelles are core-shell types formed by self-organization of the blocks in an aqueous medium or under specific experimental conditions. They possess a segment that has physical or chemical properties responding to small changes in environmental pH. The segment induces to the fast release kinetics from the micelles at tumor sites by particle shrunk/disruption. Furthermore, it can alter the biodistribution of the micelles and the interactions with tissues and cells by utilizing small pH changes. Such properties lead to overcome the problems associated with free chemo-agents, such as nonspecific toxicity, lack of tumor selectivity, and the development of multidrug resistance in various tumor cells. Therefore, the micelles have been considered as promising anti-cancer drug carriers. This review summarizes the recent progress in pH-sensitive micelles for tumor chemotherapy, particularly for those responding to tumor pH and endosomal/lysosomal pH for the treatment of multidrug resistance (MDR).