As highlighted by recent approvals, engineered antibodies now represent over 30% of biopharmaceuticals in clinical trials. Different antibody-based reagents have gained prominence in the pharmaceutical market. Based on latest recombinant technologies, the selection, humanization, and production of antibodies has been revolutionized for very diverse purposes. Intact antibody molecules can be reduced in size, rebuilt into multivalent molecules, and fused with diverse payloads, such as radionuclides, drugs, toxins, enzymes, liposomes, viruses, and vaccine-inducing epitopes.