**1. Introduction**

This chapter will focus on presenting a review of the current situation regarding the use of radioiodine labeled agents in clinical and preclinical nuclear medicine imaging and radionuclide therapy. We will show the actual clinical applications and summarize the preclinical and clinical research efforts undergoing today in this dynamic field of medicine. These agents were found to be interesting since they can be applied to both imaging the disease and for therapy, by delivering a will localized radiation dose to a target tissues or tumor volume within the human anatomy. This delivery is carried out by the so called carrier systems such as, monoclonal antibodies or fragments of those and also by nanoparticles both inorganic and organic and microspheres. These carriers will carry the radioactivity of the radionuclide to the targeted biological site. There are two types of targeting the first is direct targeting, when the pharmaceutical accumulation in a tissue or site is done through inherent pathophysiological characteristics; or

indirect which occurs if the used carries possess higher affinity to bind to a particular cell type or tissue. The good example of such radiopharmaceutical is tositumomab (131I-labeled anti-CD20 antibody), which received Food and Drug Administration (FDA) approval for the treatment of Non-Hodgkin's lymphoma in 2003.
