R v Mohan, 1994 CanLII 80, [1994] 2 SCR 9 is a leading Supreme Court of Canada decision on the use of expert witnesses in trial testimony.

Hill intended to testify that the perpetrator of the offence must have possessed several abnormal characteristics, which Mohan did not have.

Expert evidence, stated Sopinka in the ruling, should be admitted based on four criteria: Relevance is a question of law and so is determined by the judge.

In that regard, Sopinka stated: Expert evidence, to be necessary, must likely be outside the experience and knowledge of a judge or jury and must be assessed in light of its potential to distort the fact‑finding process.

In the current case, Sopinka found that there was insufficient evidence to suggest that there was any clear standard for determining the profile of a pedophile or psychopath.