The delivery and control over the release of hydrophobic imaging markers for cancer diagnosis or pharmaceutical agents for targeted therapy are of considerable interest. Nano-sized pH-sensitive polymeric micelles that rely on the enhanced permeability and retention (EPR) of vasculature and the low-pH microenvironment in cancer tissue are emerging as stimuli-responsive targeted therapies that can simultaneously release diagnostic and therapeutic agents into a cancerous region. This review focuses on the developments of pH-sensitive polymeric micelles and their biomedical applications in cancer diagnosis and targeted therapy.