The Court in recent decades has looked to the Tenth Amendment as a basis for building "the new federalism."  At other times, the Court seems to have viewed the Amendment as little more than a truism.  The meaning of the Tenth Amendment remains controversial both within the Court and among politicians, some of whom see it as the most important of all the first ten amendments.

In 1976, in National League of Cities v Usery, the Court struck down provisions of a federal law that extended minimum wage protection to almost all state and municipal employees.  Although the Court recognized that the Commerce Clause authorized minimum wage laws covering most employees, the Court held that the Constitution's structure and the Tenth Amendment's recognition of the role of states in the federal structure prohibited the federal government from telling states what they must pay their own employees. 

Nine years later, however, the Court overruled National League of Cities in Garcia v San Antonio Metro Transit Authority when Justice Blackmun--who had provided the critical fifth vote in the earlier case--reversed himself, concluding that the National League of Cities rule was causing so much confusion in the lower courts as to be "unworkable." 

The federalism debate again resurfaced in the 1992 case of New York v United States, when the Court invalidated a provision of the Radioactive Waste Policy Act that required states failing to develop an adequate plan for disposing of waste generated within their own borders to, "at the request of the owner or generator of the waste, take title to the waste."  The Court found that the so-called "take title" provision effectively "commandered the legislative processes" of states--something that the federal government cannot constitutionally do. 

In Printz v United States (1997), the Court again found that Congress had unconstitutionally intruded upon state sovereignty.  The law in question in Printz was a provision of the Brady Act requiring chief law enforcement officers of states to run background checks on prospective hand gun purchasers.  The Court rejected the federal government's argument that it could coercively enlist states in enforcing federal law, even though it might be unconstitutional to require states to make law--the problem identified in New York v U. S. 

The Court, in both New York and Printz, was not crystal clear about the basis for its decisions.  The cases could be seen either as resting on the 10th Amendment itself or be read as concluding that the 10th Amendment limited what might otherwise have been actions within the Commerce Clause power.  More likely still, the cases should be viewed as concluding that the federal actions amounted to violations of federalism principles implicit in the structure of the Constitution (including, of course, the 10th Amendment).

The Tenth Amendment was cited for support by the Court in its 2013 decision invalidating a key portion of the Voting Rights Act, Shelby County v Holder.  In its 5 to 4 decision invalidating the Act's coverage formula, which required a group of mostly southern states to seek preclearance from the federal government for changes in their voting laws, the Court noted that the Tenth Amendment was intended to give states "broad autonomy in pursuing legislative objectives."  Mostly, however, the decision rested on the principle that states under our Constitution enjoy "equal sovereignty."  The Court used the phrase repeatedly, including in the first paragraph of the opinion, and stressed that equal sovereignty was a "historic tradition" and "essential to the scheme upon which the Republic was organized."  The takeaway from the case is that differential treatment of the states by the federal government, at least with respect to matters relating to its ability to pursue its legislative objectives, will be subject to heightened judicial scrutiny--burdens will have to be justified by a substantial federal interest and there must be a substantial link between the states singled out for less favorable treatment and the problem addressed.  If, for example, the Congress were to use its Commerce Clause power to require 15 states to have their laws imposing taxes on Internet sales to residents be precleared by the Commerce Department, but left the other 35 free to enact such legislation without federal approval, such a law would almost certainly be struck down under Shelby's "equal sovereignty principle."