Antibodies have been conjugated to radionuclides for various in vitro and in vivo applications. Radiolabeled antibodies have been used in clinics and research for diagnostic applications both in vitro as reagents in bioassays and in vivo as imaging agents. Further, radiolabeled antibodies are used as direct therapeutic agents for cancer radioimmunotherapy or as tracers for studying the pharmacokinetics and biodistribution of therapeutic antibodies. Antibodies are labeled with radiohalogens or radiometals, and the choice of candidate radionuclides for a given application is dictated by their emission range and half-life. The conjugation chemistry for the coupling of MAbs with the radiometals requires a chelator, whereas radiohalogens can be incorporated directly in the antibody backbone. In this chapter, we describe the commonly used methods for radiolabeling and characterizing the antibodies most commonly used radiohalogens (125I/131I) and radiometals (177Lu/99mTc).