A variety of organic and inorganic nanomaterials with dimensions below several hundred nanometers are recently emerging as promising tools for cancer therapeutic and diagnostic applications due to their unique characteristics of passive tumor targeting. A wide range of nanomedicine platforms such as polymeric micelles, liposomes, dendrimers, and polymeric nanoparticles have been extensively explored for targeted delivery of anti-cancer agents, because they can accumulate in the solid tumor site via leaky tumor vascular structures, thereby selectively delivering therapeutic payloads into the desired tumor tissue. In recent years, nanoscale delivery vehicles for small interfering RNA (siRNA) have been also developed as effective therapeutic approaches to treat cancer. Furthermore, rationally designed multi-functional surface modification of these nanomaterials with cancer targeting moieties, protective polymers, and imaging agents can lead to fabrication versatile theragnostic nanosystems that allow simultaneous cancer therapy and diagnosis. This review highlights the current state and future prospects of diverse biomedical nanomaterials for cancer therapy and imaging.