Immunotherapy of cancer has been explored for over a century, but it is only in the last decade that various antibody-based products have been introduced into the management of patients with diverse cancers. At present, this is one of the most active areas of clinical research, with eight therapeutic products already approved in oncology. Antibodies against tumor-associated markers have been a part of medical practice in immunohistology and in vitro immunoassays for several decades, have even been used as radioconjugates in diagnostic imaging, and are now becoming increasingly recognized as important biological agents for the detection and treatment of cancer. Molecular engineering has improved the prospects for such antibody-based therapeutics, resulting in different constructs and humanized/human antibodies that can be administered frequently. Consequently, a renewed interest in the development of antibodies conjugated with radionuclides, drugs, and toxins has emerged. We review how antibodies and immunoconjugates have influenced cancer detection and therapy, and also describe promising new developments and challenges for broader applications.