In the past five years therapeutic monoclonal antibodies have established themselves as perhaps the most important and rapidly expanding class of therapeutic drugs. More than 25% of pharmacological agents that are currently under development are based on antibodies and the total income generated from them in 2002 exceeded $3 billion, and is predicted to rise to $10–20 billion by 2010. Many experts feel that antibodies directed at cancer targets are likely to dominate the market for the foreseeable future. In this review, we will discuss some of the factors that, after more than 25 years of development, have led to this transformation in the antibody field.