Federal law originates with the Constitution, which gives Congress the power to enact statutes for certain limited purposes like regulating interstate commerce. Nearly all statutes have been codified in the United States Code. Many statutes give executive branch agencies the power to create regulations, which are published in the Federal Register and codified into the Code of Federal Regulations. Regulations generally also carry the force of law under the Chevron doctrine. Many lawsuits turn on the meaning of a federal statute or regulation, and judicial interpretations of such meaning carry legal force under the principle of stare decisis.

In the beginning, federal law traditionally focused on areas where there was an express grant of power to the federal government in the federal Constitution, like the military, money, foreign affairs (especially international treaties), tariffs, intellectual property (specifically patents and copyrights), and mail. Since the start of the 20th century, aggressive interpretations of the Commerce and Spending Clauses of the Constitution have enabled federal law to expand into areas like aviation, telecommunications, railroads, pharmaceuticals, antitrust, and trademarks. In some areas, like aviation and railroads, the federal government has developed a comprehensive scheme that preempts virtually all state law, while in others, like family law, a relatively small number of federal statutes (generally covering interstate and international situations) interacts with a much larger body of state law. In areas like antitrust and trademark, there are powerful laws at both the federal and state levels that coexist with each other. In a handful of areas like insurance, Congress has enacted laws expressly refusing to regulate them as long as the states have laws regulating them (see, e.g., the McCarran-Ferguson Act).

Under the doctrine of Erie Railroad Co. v. Tompkins (1938), there is no general federal common law. Although federal courts can create federal common law in the form of case law, such law must be linked one way or another to the interpretation of a particular federal constitutional provision, statute, or regulation (which in turn was enacted as part of the Constitution or after). Federal courts lack the plenary power possessed by state courts to simply make up law, which the latter are able to do in the absence of constitutional or statutory provisions replacing the common law. Only in a few narrow limited areas like admiralty law has the Constitution expressly authorized the continuation of English common law at the federal level (meaning that in those areas federal courts can continue to make law as they see fit, subject to the limitations of stare decisis).