The Constitution Act, 1867 contains no express provision on federal or provincial treaty-making powers. These powers were reserved to the Imperial government at the time when the Constitution Act, 1867 was adopted by the Imperial Parliament. The Constitution Act, 1867 also contained only one provision that dealt with the implementation of Imperial treaty obligations in the Canadian federation and that provision is now obsolete. Because Canada's gradual autonomy from the British Empire was not accompanied by a thorough modification of the text of the Canadian constitution, nothing has been expressly provided in relation to treaty powers in the Canadian federation. Canadian constitutional law dealing with treaty powers is therefore a pure product of the Canadian "organic constitutionalism" tradition. This thesis examines this form of constitutionalism through the specific case of the treaty powers in Canada. In particular, this study hopes to deepen our understanding of the multiple legal consequences of the constitutionally entrenched principle of federalism recognized by the Supreme Court of Canada in the Reference re Secession of Quebec, [1998] 2 S.C.R. 217. More specifically, the thesis presents an in-depth analysis of Canada (A.G.) v. Ontario (A.G.), [1937] A.C. 326 (the Labour Conventions case) where the Privy Council decided that the federal executive power could sign and ratify treaties in the name of Canada but that the implementation of treaty obligations - when legislative action is required - is the responsibility of the legislature that has jurisdiction over the obligations' subject-matters. The Privy Council did not specify, however, which institution(s) has the power to conclude treaties in relation to provincial subject-matters. This thesis tackles this problem. It is demonstrated in this thesis that no rule or principle of Canadian constitutional law nor of international law grants plenary and exclusive treaty-making powers to the federal executive.