Canada's Constitution is not a single document as in the United States. It is made up of acts of the British and Canadian Parliaments, as well as legislation, judicial decisions and agreements between the federal and provincial governments.

It also includes unwritten elements such as British constitutional conventions, established custom, tradition and precedent. Responsible government, for example, in which the Cabinet is collectively responsible to the elected House of Commons and must resign if it loses a vote of confidence, is a fundamental, but unwritten, element of Canadian parliamentary democracy at the federal and provincial levels.

The Constitution's basic written foundations are the Constitution Act, 1867, which created a federation of four provinces — Ontario, Quebec, Nova Scotia and New Brunswick — under the British Crown, and the Constitution Act, 1982, which transferred formal control over the Constitution from Britain to Canada and entrenched a Canadian Charter of Rights and Freedoms and procedures for constitutional amendment.

The Constitution Act, 1867 (formerly known as the British North America Act) contains the fundamental distribution of powers between the federal and provincial governments.

The founders of Canada were determined to create a strong central government while, at the same time, assigning substantive powers to provincial legislatures that would enable them to maintain their identity, culture and institutions. They gave the federal Parliament jurisdiction over defence and foreign policy, trade, transportation, communications and Indians and Indian lands. Parliament was also granted certain extraordinary powers, such as the broad authority "to make laws for the peace, order and good government of Canada," the right to disallow provincial legislation, and to declare local undertakings to be for the general advantage and thus to fall under federal jurisdiction (for example, the regulation of sales of alcohol or firearms).

In a deliberate departure from the U.S. model, the Fathers of Confederation awarded to the federal Parliament the residual power, i.e., jurisdiction over all areas not specifically assigned to the provincial legislatures. (The 10th Amendment to the U.S. Constitution reserves to the states or to the people all powers not specifically enumerated.) The federal government was also provided with unlimited taxing powers while the provinces were restricted to direct taxes within the province.

The provincial legislatures were given authority over education, hospitals, property, civil rights, natural resources and other "local works and undertakings." Jurisdiction over two areas, immigration and agriculture, was divided between the two levels of government.

The basic distribution has been formally amended only four times since 1867: amendments in 1940, 1951 and 1964 gave the federal government responsibility for unemployment insurance, old age pensions and supplementary benefits; and the Constitution Act, 1982 strengthened provincial control over natural resources and affirmed other protections, such as for Canada's Aboriginal peoples and for regional development. The balance of power has shifted back and forth, however, through a combination of judicial decisions, circumstances and political compromise.

The Judicial Committee of the Privy Council of Great Britain was the court of final appeal for Canada until 1949 when full authority was transferred to the Supreme Court of Canada. It rendered some 120 decisions on the distribution of legislative powers and established the fundamental principle that the provinces and the federal government are sovereign in their areas of jurisdiction. In general, the Committee contributed to the decentralization of the Canadian federation through its broad interpretation of provincial powers, particularly over property and civil rights, and its relatively narrow reading of the principal federal powers. (In contrast, the U.S. judiciary played a significant role in centralizing power under the federal government through its broad interpretation of federal jurisdiction over interstate commerce.)

The Constitution Act, 1867 contains specific provisions designed to protect the distinctiveness of Quebec. It recognizes Quebec's civil code as distinct from the English common law in effect in the other provinces, and provides for the use of English and French in Parliament and the Quebec legislature and courts, and for publicly-funded separate schools for Protestant and Catholic minorities in Quebec and Ontario (and later in Manitoba, Saskatchewan and Alberta).