Disallowance and reservation are historical constitutional powers in Canada that act as a mechanism to delay or overrule legislation passed by Parliament or a provincial legislature. In contemporary Canadian history, disallowance is an authority granted to the governor general in council (federal cabinet) to invalidate an act (also called a "statute") passed by a provincial legislature. Reservation is an authority granted to the lieutenant governor to withhold royal assent from a bill which has been passed by a provincial legislature; the bill is then "reserved" for consideration by the federal cabinet.

In Canadian constitutional law, the powers of reservation and disallowance of federal legislation formally remain in place in section 55 and section 56 of the Constitution Act, 1867, and are extended to provincial legislation by section 90. The initial intent of disallowance, and its practice for the first few years of Confederation, was considered a means of ensuring parliament enacted legislation compliant with the constitution.

Since Confederation in 1867, the Government of the United Kingdom has only disallowed one federal law, while the government of Canada has disallowed 112 provincial laws, with the most recent instance occurring in 1943 when Alberta's law that limited land sales to Hutterites and other "enemy aliens" was invalidated. The power of reservation has been exercised 21 times by the governor general, all before 1878, and 70 times by various lieutenant governors, with the most recent case in Saskatchewan in 1961 when the lieutenant governor reserved assent on a bill related to mining contracts.

Canada is a federation with eleven components: the national Government of Canada and ten provincial governments. These eleven governments derive their authority from the Constitution of Canada. There are also three territorial governments in the far north, which exercise powers delegated by the federal Parliament. Each jurisdiction is generally independent from the others in its realm of legislative authority. The division of powers between the federal government and the provincial governments is based on the principle of exhaustive distribution: all legal issues are assigned to either the federal Parliament or the provincial legislatures. In Canada, Parliament is composed of two legislative chambers, the elected House of Commons and the appointed Senate, and together with the Governor General acting as the representative of the Crown. In provinces, the legislature is composed of one chamber, an elected group of representatives, overseen by the Lieutenant Governor appointed by the Governor General on the advice of the federal government, with the duty of acting as the representative of the Crown for the province. Once a bill has passed through the chamber(s), the bill is presented to the governor-general (or lieutenant governor) for assent on the Sovereign's behalf.

Disallowance is the decision by a viceregal representative of the Crown to veto an act of the Parliament of Canada, or a provincial legislature, and the act ceases to operate as law. The authority to disallow an act of the federal Parliament was set out in section 56 of the Constitution Act, 1867, and was held by the Crown in council. As part of the development of the modern Commonwealth of Nations reforms from 1926 to 1931 recognized that constitutional convention dictated that the government of the United Kingdom was not capable of utilizing disallowance without the approval of the Canadian federal cabinet.

Section 56: Where the Governor General assents to a Bill in the Queen's Name, he shall by the first convenient Opportunity send an authentic Copy of the Act to One of Her Majesty's Principal Secretaries of State, and if the Queen in Council within Two Years after Receipt thereof by the Secretary of State thinks fit to disallow the Act, such Disallowance (with a Certificate of the Secretary of State of the Day on which the Act was received by him) being signified by the Governor General, by Speech or Message to each of the Houses of the Parliament or by Proclamation, shall annul the Act from and after the Day of such Signification.

The authority to disallow an act of a provincial legislature is set out in section 90 of the Constitution Act, 1867 and held by the governor general acting on the advice of the Canadian federal cabinet (e.g., governor general in council). The decision to disallow a provincial act must be made within one year of the governor general's receipt of the act from the lieutenant governor.

Section 90: The following Provisions of this Act respecting the Parliament of Canada, namely, — the Provisions relating to Appropriation and Tax Bills, the Recommendation of Money Votes, the Assent to Bills, the Disallowance of Acts, and the Signification of Pleasure on Bills reserved, — shall extend and apply to the Legislatures of the several Provinces as if those Provisions were here re-enacted and made applicable in Terms to the respective Provinces and the Legislatures thereof, with the Substitution of the Lieutenant Governor of the Province for the Governor General, of the Governor General for the Queen and for a Secretary of State, of One Year for Two Years, and of the Province for Canada.