Employing theranostic nanoparticles, which combine both therapeutic and diagnostic capabilities in one dose, has promise to propel the biomedical field toward personalized medicine. This review presents an overview of different theranostic strategies developed for the diagnosis and treatment of disease, with an emphasis on cancer. Herein, therapeutic strategies such as nucleic acid delivery, chemotherapy, hyperthermia (photothermal ablation), photodynamic, and radiation therapy are combined with one or more imaging functionalities for both in vitro and in vivo studies. Different imaging probes, such as MRI contrast agents (T(1) and T(2) agents), fluorescent markers (organic dyes and inorganic quantum dots), and nuclear imaging agents (PET/SPECT agents), can be decorated onto therapeutic agents or therapeutic delivery vehicles in order to facilitate their imaging and, in so doing, gain information about the trafficking pathway, kinetics of delivery, and therapeutic efficacy; several such strategies are outlined. The creative approaches being developed for these classes of therapies and imaging modalities are discussed, and the recent developments in this field along with examples of technologies that hold promise for the future of cancer medicine are highlighted.