Can I use an expert witness in an Ontario civil lawsuit and how does it work?

Yes, expert witnesses play an important role in many Ontario civil cases. Where a fact or opinion is beyond the knowledge of an ordinary person — such as the cause of a building failure, the standard of care in a medical procedure, or the valuation of a business — an expert can provide opinion evidence. Ordinary witnesses are generally limited to facts they personally observed.

In Ontario Superior Court, expert witnesses must provide a written report in advance of trial that sets out their opinions and the basis for them. The report must be served on the other side within a deadline set by the court or agreed between the parties. The opposing party can retain their own expert and file a responding report.

Courts have gatekeeping rules about expert evidence. The expert must be qualified in the relevant field, and the opinion must be based on proper methodology. An expert's primary duty is to the court — not to the party who retained them — and courts scrutinize whether an expert is truly objective. Retaining the right expert early in your case can be critical to establishing or defending against technical claims.