(b)  The Rules-especially Rule 702-place appropriate limits on
the admissibility of purportedly scientific evidence by assigning
to the trial judge the task of ensuring that an expert's testimony
both rests on a reliable foundation and is relevant to the task at
hand.  The reliability standard is established by Rule 702's
requirement that an expert's testimony pertain to ``scientific ...
knowledge,'' since the adjective ``scientific'' implies a
grounding in science's methods and procedures, while the word
``knowledge'' connotes a body of known facts or of ideas inferred
from such facts or accepted as true on good grounds.  The Rule's
requirement that the testimony ``assist the trier of fact to
understand the evidence or to determine a fact in issue'' goes
primarily to relevance by demanding a valid scientific connection
to the pertinent inquiry as a precondition to admissibility. Pp.
9-12.
   (c)  Faced with a proffer of expert scientific testimony under
Rule 702, the trial judge, pursuant to Rule 104(a), must make a
preliminary assessment of whether the testimony's underlying
reasoning or methodology is scientifically valid and properly can
be applied to the facts at issue.  Many considerations will bear
on the inquiry, including whether the theory or technique in
question can be (and has been) tested, whether it has been
subjected to peer review and publication, its known or potential
error rate, and the existence and maintenance of standards
controlling its operation, and whether it has attracted widespread
acceptance within a relevant scientific community.  The inquiry is
a flexible one, and its focus must be solely on principles and
methodology, not on the conclusions that they generate.
Throughout, the judge should also be mindful of other applicable
Rules.  Pp. 12-15.
   (d)  Cross-examination, presentation of contrary evidence, and
careful instruction on the burden of proof, rather than wholesale
exclusion under an uncompromising ``general acceptance'' standard,
is the appropriate means by which evidence based on valid
principles may be challenged.  That even limited screening by the
trial judge, on occasion, will prevent the jury from hearing of
authentic scientific breakthroughs is simply a consequence of the
fact that the Rules are not designed to seek cosmic understanding
but, rather, to resolve legal disputes.  Pp. 15-17. 951 F. 2d 1128,
vacated and remanded.
  Blackmun, J., delivered the opinion for a unanimous Court with
respect to Parts I and II-A, and the opinion of the Court with
respect to Parts II-B, II-C, III, and IV, in which White, O'Connor,
Scalia, Kennedy, Souter, and Thomas, JJ., joined.  Rehnquist, C.
J., filed an opinion concurring in part and dissenting in part, in
which Stevens, J., joined.
---
---------------