What does it mean that the Supreme Court of Canada interprets the Constitution?

The Supreme Court of Canada has the final authority to determine the meaning and application of Canadian constitutional documents. This constitutional-interpretation function flows from the Court's role as Canada's final court of appeal (since 1949, when appeals to the Judicial Committee of the Privy Council in London were abolished) and from section 52(1) of the Constitution Act, 1982, which provides that the Constitution is the supreme law of Canada and that any law inconsistent with the Constitution is, to the extent of the inconsistency, of no force or effect.

The Court's constitutional-interpretation role covers four main areas. First, the federal-provincial division of powers under sections 91 and 92 of the Constitution Act, 1867. The Court decides whether a particular law falls within federal or provincial jurisdiction (with cases such as the Reference re Securities Act of 2011 and the Reference re Greenhouse Gas Pollution Pricing Act of 2021). Second, the Canadian Charter of Rights and Freedoms (1982), with the Court having decided more than 600 Charter cases since 1982. Third, section 35 of the Constitution Act, 1982 protecting Indigenous rights, with leading cases including R. v. Sparrow (1990), Delgamuukw (1997), and Tsilhqot'in (2014). Fourth, constitutional amendment procedures and conventions, with leading cases including the Patriation Reference (1981) and the Reference re Senate Reform (2014).

The Court's constitutional rulings can take three forms. Decisions in concrete cases (where a real litigant challenges a law on constitutional grounds) bind all lower courts in Canada. Reference cases (where the federal government submits questions to the Court under section 53 of the Supreme Court Act) produce advisory opinions that, while technically non-binding, are nearly always followed in practice. Provincial governments can submit reference questions to their provincial Court of Appeal, which can then be appealed to the Supreme Court of Canada.

The Supreme Court's constitutional-interpretation role makes it one of the most powerful courts in the world. The Court can strike down federal and provincial legislation that violates the Constitution. The Court has shaped Canadian law on abortion (R. v. Morgentaler, 1988), assisted dying (Carter v. Canada, 2015), Aboriginal title (Tsilhqot'in, 2014), language rights (Mahe v. Alberta, 1990), gender equality (Andrews v. Law Society, 1989), criminal procedure (R. v. Jordan, 2016), and many other areas. The Court's constitutional rulings can be modified only by constitutional amendment (which requires the agreement of Parliament and seven provinces representing 50 per cent of the population, or unanimous consent for certain matters under the amending formula). Section 33 of the Charter (the notwithstanding clause) allows Parliament or provincial legislatures to override certain Court rulings on Charter rights for renewable five-year periods.