Because the factual truths at issue in a case often go beyond what the average juror can be expected to know, courts have come increasingly to rely on expert witnesses, among them scientists from just about every specialty you can think of: experts on blood, bullets, bite-marks, battered wives; on PCBs, paternity, poisons, post-traumatic stress; on radon, recovered memories, rape trauma syndrome, random DNA-match probabilities; on psychosis, asbestosis, silicosis (and for all I know, on psittacosis!). But as long as courts have relied significantly on scientific witnesses, there have been complaints: about the scientific ignorance and gullibility of attorneys, judges, and jurors; about “witness-shopping”; and–as my opening quotation illustrates–about the irresponsibility and venality of professional scientific experts willing to say whatever is needed to advance the cause of the party that hires them. And as reliance on expert witnesses has grown, so has the felt need for courts to ensure that the expert testimony admitted is not just flimsy or interested speculation, but reliable enough to be more helpful than misleading.