This is when the federal government comes in.
Often, if an area of law or regulation is not clearly assigned to either the provinces or the federal government, it is left up to the discretion of the federal government. However, as new areas of the law emerge, deciding whether these new heads of power properly belong to either the federal or provincial government is often a difficult distinction to make. Governments have therefore relied regularly on the judiciary (the Courts) to make a final ruling.
In Canada, the courts are known as the independent judiciary and are expected, and able, to come to their own conclusions, despite what the other branches of government have done. It is this independence that has allowed the courts to play an important role as the decision maker, or tiebreaker, when debates about the federal division of powers arise. The courts have often been responsible for ensuring that neither the federal nor the provincial governments are overstepping their role.  In making these decisions, the courts have had to rely on and interpret a number of constitutional principles.
For example, Section 92A (3) of the Constitution Act, 1982 states that provincial and federal regulation of natural resources can exist together so long as the provincial law does not directly interfere with the federal law’s main purpose. If and when that interference happens, the provincial law is considered invalid. 
The courts have also played a role in determining which activities fall under the residual federal power, found in the peace, order and good government clause in Section 91 of the Constitution.  This clause gives the federal government control over national emergencies or matters considered to have a “national dimension”. In particular, as new areas of environmental law have emerged, the governments and the courts have had to determine whether they can be controlled by the federal government under one of the overarching sections, such as peace, order and good government, or whether they should be allocated to the provinces.
For example, over time, unallocated powers such as control over aeronautics, radio, and official languages have gone to the federal government rather than to the provinces, under the peace, order and good government power. Further, through the lens of having “national dimensions”, certain matters – including those originally of a local nature and under provincial power (e.g., day-to-day health care) – can, through changed circumstances, such as an epidemic, become subject to federal jurisdiction. 
There are also other concurrent powers or areas of law that are exercisable by both jurisdictions – meaning that both the provincial and federal governments can make rules about the area of law in question. In those cases, the court may be called upon to distinguish whether a provincial law is intruding upon the main area of a federal law or whether it is a situation where both laws can exist side by side.
 Patrick J Monahan & Byron Shaw, Constitutional Law, 3d ed (Toronto: Irwin Law, 2013) at 10.
 Constitution Act, 1867, (UK), 30 & 31 Vict, c 3, reprinted in RSC 1985, App II, No 5, s 92(A)(3).
 Constitution Act, 1867, s 91.
 W.H. McConnell & Richard Foot, “Constitution Act, 1867” (30 January 2015) The Canadian Encyclopedia online: http://www.thecanadianencyclopedia.ca/en/article/constitution-act-1867/; Gerald A. Beaudoin & Jon Tattrie, “Constitutional Law” (6 July 2015) The Canadian Encyclopedia online: http://www.thecanadianencyclopedia.ca/en/article/constitutional-law/.
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