Theranostics is the definition of bringing the imaging agent and therapeutic drug together in the same delivery design. The term ‘theranostic’ was first defined by John Funkhouser in 2002 and since then it became one of the most attractive fields in treatment of severe diseases. Nanoparticles (NPs) are the most suitable carrier systems due to their plasmonic and magnetic properties, active surface areas and various physicochemical properties. Development of therapeutic NPs provide both active and passive targeting, sensitive monitoring of biological circulation, effective drug carrying and releasing, longer circulation time and efficient clearance from renal system. Here in this chapter, we discussed commonly used cancer treatment theranostic NPs that utilize imaging modalities such as magnetic resonance imaging (MRI), radionuclidebased imaging; positron emission tomography (PET) and single-photon emission computed tomography (SPECT) and X-ray-computed tomography (CT).