In the vast majority of cases, committees are able to obtain the evidence they seek by inviting witnesses to appear before them. However, if a witness has declined an invitation to appear, a committee may issue a summons to that witness by adopting a motion to that effect. If a proposed witness fails to appear when summoned, the committee may report the fact to the House. The House then takes any action it deems appropriate."
This document is available at http://www.parl.gc.ca/information/about/process/house/WitnessesGuides/Witness-e.htm