There has been a paradigm shift in the way biology is carried out. This shift is being driven by several enabling technologies: genomics, which offers a virtually unlimited number of new gene products to study; combinatorial chemistry, which promises an almost unlimited number of compounds to act as potential agonists and antagonists; and the advent of genomewide experiments such as structural genomics, which are aimed at obtaining some functional information about as many gene products as possible. In practice, however, there are limitations to the paradigm. These limitations come in large part from the fact that the term "function" means many things, and its meaning changes depending on who is asking the question and what sorts of experiments are being employed to probe it.