**Abstract**

Malignancy and many inflammatory diseases have become a major concern for mankind over the years. The conventional therapy of these diseases lacks the effectiveness of the better diagnosis and targeted treatment of these diseases, but nuclear medicine can be regarded as a savior in the current scenario. Over the years, radioactivity of radioisotopes has been employed for treatment of many diseases. Nuclear medicines came up with radiopharmaceuticals that impart the ability to destroy specific diseased cells with high-energy-emitting radionuclides. Moreover, the emergence of theranostics, which is a combination of single drug used both for diagnostic as well as therapeutic purpose, has added a new feather in the field of nuclear medicines for providing a specific and personalized treatment to the patient. The current chapter discusses about techniques used for imaging of these radionuclides for better therapy and diagnosis of the root cause of the concerned disease by positron emission tomography (PET)/CT and single photon emission computed tomography (SPECT)/CT as well as the advantages and disadvantages associated with them. It also describes about applications of theranostics and nuclear imaging in cancer treatment and their future perspective.

**Keywords:** radionuclides, nuclear medicines, nuclear imaging, theranostics, radiopharmaceuticals
