The governments' different areas of legislative authority are set out described in [http://laws-lois.justice.gc.ca/eng/Const/page-4.html#h-18 sections 91] and [http://laws-lois.justice.gc.ca/eng/Const/page-4.html#h-19 92] of the ''[http://laws-lois.justice.gc.ca/eng/Const/page-1.html Constitution Act, 1867]''. The federal government can only make laws about the subjects set out in section 91, and the provincial governments can only make laws about the subjects set out in section 92.

From a family law perspective, this means that only the federal government has the authority to make laws about marriage and divorce, while the provincial governments have the exclusive authority to make laws about marriage ceremonies, the division of property, and civil rights. As a result, the federal ''[[Divorce Act]]'' talks about divorce and issues that are related to divorce, like the care of parenting children, child support, and spousal support, and the federal ''[http://canlii.ca/t/7w02 Civil Marriage Act]'' talks about marriage and issues related to marriage. The provincial ''[[Family Law Act]]'' talks about the care of parenting children, child support, and spousal support as well, but also talks about the division of family property and family debt, the management of children's property, the guardianship of children, and determining the parentage of children.

Sometimes the subjects over which each level of government has authority overlap and, according to a legal principle called the ''doctrine of paramountcy'', all laws are not created equal. Under this doctrine, federal legislation on a subject trumps any provincial legislation on the same subject. This is important because in family law both the ''Divorce Act'' and the ''Family Law Act'' deal with parenting children, child support , and spousal support. As a result, orders under the ''Divorce Act'' <span class="noglossary">will</span> always usually be paramount to orders under the ''Family Law Act'' on the same subject. It isn't ''quite'' right to say that federal legislation "trumps" provincial legislation. It's a little more complex, and this is important because the ''Divorce Act'' and the ''Family Law Act'' cover so many of the same subjects. Really, what the legal test says is that in order for the federal legislation to win, there must be a "functional incompatibility" between the provincial legislation and the federal legislation, so that it is impossible to comply with both statutes and that complying with one statute would frustrate the purpose of the other statute. Here's what the Supreme Court of Canada said in a 2007 case called ''[http://canlii.ca/t/1rmr1 Canadian Western Bank v Alberta]'': <blockquote>"According to the doctrine of federal paramountcy, when the operational effects of provincial legislation are incompatible with federal legislation, the federal legislation must prevail and the provincial legislation is rendered inoperative to the extent of the incompatibility. The doctrine applies not only to cases in which the provincial legislature has legislated pursuant to its ancillary power to trench on an area of federal jurisdiction, but also to situations in which the provincial legislature acts within its primary powers, and Parliament pursuant to its ancillary powers. ... The onus is on the party relying on the doctrine of federal paramountcy to demonstrate that the federal and provincial laws are in fact incompatible by establishing either that it is impossible to comply with both laws or that to apply the provincial law would frustrate the purpose of the federal law."</blockquote> That's a bit complicated, but what it all boils down to is what the Supreme Court of British Columbia said in a 2014 case called ''[http://canlii.ca/t/g6700 B.D.M. v A.E.M.]'': "the doctrine of paramountcy does not preclude consideration and application of the FLA in family law proceedings in which a divorce is granted."