While not restricted to determination of sanity, rule 704 primarily affects Federal insanity defense trials, in which courts and lawyers rely heavily on experts. At such trials, evidence typically focuses on the mental state associated with the defendant's conduct rather than on the commission of the criminal act. Psychiatric experts, trained to fit particular behaviors into a larger pattern of mental illness, can help explain the accused's cognitive capacity at the time of the criminal act. By resurrecting traditional prohibitions on expert testimony concerning ultimate issues, rule 704(b) as amended departs from the original approach to expert testimony and makes expert witnesses less useful to factfinders by encouraging incomplete and indirect testimony. It hampers both prosecution and defense in persuading factfinders, excludes useful testimony, and forces judges to devote time to difficult line-drawing problems. Further, if rules 403 and 702 are properly applied, it is unnecessary for limiting confusion from expert testimony. Judges should use their discretion and construe rule 704(b) narrowly to minimize its deletorious impact on insanity defense trials. 129 footnotes.