Partisan expert witnesses, selected, prepared, and presented by the parties, are one of the central features of Anglo-American judicial proceedings. They provide fact finders with essential technical information and are authorized to propound a range of opinions and conclusions that other witnesses are not. Their views are often the deciding factor in hard-fought cases. Yet their association with one party and their apparent partiality have long troubled legal commentators. These concerns have grown in recent years along with the perception, not based on a great deal of empirical evidence, that more and more experts are being used to prove more and more different things in modern American trials.