Can I bring a motion to quash a subpoena that was served on me in Ontario?

In an Ontario civil lawsuit, the document used to compel someone to attend and give evidence at trial is generally called a summons to witness rather than a subpoena, though people often use the terms interchangeably. If you've been served with one and believe it's improper — for example, because it demands documents or attendance that has nothing to do with the issues in the case, is unreasonably broad, was served without proper notice, or amounts to harassment rather than a genuine need for your evidence — you can bring a motion asking the court to set it aside or narrow its scope.

The court weighs whether your attendance or the documents sought are genuinely relevant to a real issue in the case, against the burden the summons places on you as a non-party or witness who isn't otherwise involved in the dispute. Courts are generally reluctant to let a summons be used as a fishing expedition or as an indirect way to harass someone. If your objection is only to part of what's demanded, the more efficient step is often negotiating a narrower scope with the party who issued it before bringing a motion, since a partial resolution can avoid the cost of a full hearing.