IntroductionOne of the greater challenges of structural biology is to characterize proteins in enough detail so that the structures can be used for computational prediction of function, and for the design of compounds that can be used to modulate or to interfere with that function. In general, the function of a protein is associated with its interaction with other molecules, either small molecules or other large biological molecules, such as nucleic acids or proteins. Invariably, the function can be disrupted when that interaction is prevented. A complete industry exists solely for the purpose of finding such disruptors -they are called drugs. The discovery of drugs had been a serendipitous process, in an age when very little was known about the structures of the small molecules or their targets. The process of discovery has become more sophisticated as such structures became available.The success of this process relies on knowing where an interaction site is located on a protein and characterizing the properties of that site. The first step is therefore the location of such a site. The properties of the site can then be used to design a molecule that fits it perfectly in terms of size, shape and chemical properties. One method to obtain such properties is to map a site with chemical probe molecules. Ideally these probe molecules can be connected to form a larger molecule that takes advantage of as many of the potential interaction partners in the site as possible. Finally, when the process of finding and characterizing becomes robust enough, the process can be accomplished computationally.At this moment in time, locating a binding site on a protein without any extraneous information, such as a fortuitously bound ligand in the crystal structure, is only marginally successful. The characterization of a binding site is still very laborious when done experimentally and is not robust when done computationally. Thus, mapping such a site with chemical probes could help characterize the site, the map then being used to design a specific ligand unique for the site. In addition, such a map can provide the data from which computational approaches can be calibrated. One way in which mapping can be achieved is to use a variety of small probe molecules and observe where they bind to the surface of the protein. 67 Fragment-based Approaches in Drug Discovery. Edited