With the availability of new biological therapies there is the need of more accurate diagnostic tools to non-invasively assess the presence of their targets. In this scenario nuclear medicine offers many radiopharmaceuticals for SPECT or PET imaging of many pathological conditions. The availability of monoclonal antibodies provides tools to target specific antigens involved in angiogenesis, cell cycle or modulation of the immune systems. The radiolabelling of such therapeutic mAbs is a promising method to evaluate the antigenic status of each cancer lesion or inflamed sites before starting the therapy. It may also allow to perform follow-up of such biological therapies. In the present review we provide an overview of the most studied radiolabelled antibodies for therapy decision making and follow-up of patients affected by cancer and other pathological conditions.