As the law's policies for the use of expertise shift from stage to stage as litigation progresses or differ between categories of legal cases (criminal vs. civil) or with a party's use of an expert (from being a nonwitness consultant to an expert witness at trial), the law expects the role of the expert to be reshaped accordingly. On some important issues, the law sends contradictory messages: What its formal rules announce is at war with its structure and practices. And these, in turn, sometimes are in tension with the demands of the expert's professional ethical codes. On other matters of importance to experts, the law is silent, because the rules were motivated by a need to control the behavior of parties and lawyers, not experts. The result of all this is to present those who would be conscientious expert witnesses with a need to resolve nearly impossible role conflicts and ethical dilemmas. 35 notes and 45 references (Author abstract)