While litigation experts hired by a party to a lawsuit must comply with the comprehensive expert report rules set out in Rule 53.03 of the Ontario Rules of Civil Procedure, participant experts such as treating health practitioners and non-party experts such as those hired by a non-party insurer don’t need to comply with the rule in order to provide expert opinion evidence at trial, according to the Ontario Court of Appeal in its pragmatic and much-anticipated decision inWesterhof v. Gee last week.