On 19 October 2005, the Supreme Court of Canada released its unanimous decisions in Canada Trustco Mortgage Company v. Canada and Mathew v. Canada - the Supreme Court's first decisions on the general anti-avoidance rule (GAAR) in Canada's Income Tax Act. The rule, enacted in response to the Supreme Court's 1984 decision in Stubart Investments, is designed to distinguish between legitimate tax planning and abusive tax avoidance. This article reviews the Supreme Court's decisions in Canada Trustco and Mathew, in which the Court considered the various issues pertaining to the GAAR and provided important guidance on its interpretation and application. The article outlines the facts of each case and the grounds on which the Crown sought to apply the GAAR. The article then summarizes the decisions of the Tax Court of Canada and the Federal Court of Appeal in each case. The article also examines the Supreme Court's decisions and considers the Court's general approach to tax law and the GAAR and its application of this approach to the facts of each case. The last part of the article summarizes the conclusions of this examination.