The introduction of a new therapeutic modality, as happened with the arrival of monoclonal antibodies on the market in the 1980s, can change the pharmaceutical landscape. The introduction of such a new product class inevitably created some technical challenges, e.g. overcoming in vivo immunogenicity, but solutions were eventually found and recombinant antibodies are now a major class of biopharmaceuticals. The high specificities and affinities of antibodies have led to their use both as therapeutics in their own right and, to a lesser extent, as a targeting molecule for other effector molecules such as radioisotopes and cytotoxic drugs. The linkage of chemical effector entities to proteins creates major development problems in a regulated environment, many of which remain to be solved. Nanomedicine has and will produce major benefits to patients, but it is important that researchers understand the essential requirements for translation to drugs. Copyright (C) 2011 S. Karger AG, Basel