The last three decades have provided opportunities to explore the potential of treating malignant diseases with antibodies or other targeting molecules labelled with nuclides. The expanding array of new targeting molecules (recombinant antibodies or peptides) may increase the therapeutic efficacy. The title of this book "Targeted Radionuclide Tumor Therapy - Biological aspects" was selected to reinforce the concept that a major focus was devoted to understanding the biological effects of targeting and radiation. Furthermore, our rapidly expanding knowledge of low dose-rate effects, different types of cell death, autosensitization and the increasingly likely existence of cancer stem cells suggests to us that even more efficient approaches in targeting might be possible in the future. The development of targeted therapy is a true multidisciplinary enterprise involving physician scientists from the fields of nuclear medicine, radiation therapy, diagnostic radiology, surgery, gynaecology, pathology and medical oncology/haematology. It also involves many preclinical scientists working with experimental animal models, immunochemistry, recombinant antibody technologies, radiochemistry, radiation physics and basic cell biology. It is the ambition of the editors to enable deeper insights in the process of improving targeted therapy. We believe that the time now has come when targeted therapy can soon be added to standard oncology treatment regimens.