Title 26 of the U.S. Code contains nearly all of the federal tax laws. This title is commonly referred to as the "Internal Revenue Code" (IRC) or sometimes simply as "the Code."

The current version is the Internal Revenue Code of 1986, as amended. The IRC has had three major amendments: the 1939 Code, the 1954 Code, and the 1986 Code. Prior to the 1939 Code, federal tax laws were enacted in individual revenue acts.

For more information about accessing and researching the U.S Code, please see:

Section 7805 of the IRC grants the Treasury Department authority to interpret and administer the Internal Revenue Code. The Internal Revenue Service (IRS), a division of the Treasury Department, is charged with the day-to-day operations. Regulations are the highest administrative authority issued by the Treasury Department.

Treasury regulations are published in the Federal Register and codified in Title 26 of the Code of Federal Regulations (CFR). Title 26 of the CFR is updated annually on April 1. Unlike other federal regulations, which are cited to the CFR, Treasury regulations are not cited to the CFR, but instead are cited as, for example, Treas. Reg. §1.61-1.

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A complication in researching court opinions on tax law is that in the federal system, tax cases can be initially litigated in three different courts: the United States Tax Court, the U.S. District Courts, and the Court of Federal Claims.

Tax cases tried in all of these courts are later appealed to the U.S. Courts of Appeals. Although it is a national court, the Tax Court hears cases in different locations around the country, and appeals of Tax Court cases are heard by the U.S. Court of Appeals for the circuit in which the taxpayer resides. Appeals of cases from the U.S. District Courts are heard by the U.S. Court of Appeals for the circuit in which they sit. Appeals from cases decided by the U.S. Court of Federal Claims are heard solely by the U.S. Court of Appeals for the Federal Circuit.

As with all federal cases, the court of final appeal is the U.S. Supreme Court.

The United States Tax Court issues three types of decisions, and each type is published in a different resource:

1. Summary decisions,
2. Regular decisions, and
3. Memorandum decisions.

Summary decisions are issued in "small" cases, which result from an expedited procedure brought by individual taxpayers with less than $10,000 in tax liability. Summary decisions are not appealable, and thus are not considered precedent. Summary decisions are not officially published, but may appear in some comprehensive legal databases.

Regular decisions are decisions involving new or unusual points of law, and they are decided by the entire Tax Court en banc, meaning that they are decided by all of the Tax Court's judges. Regular decisions are officially published in the Reports of the United States Tax Court (T.C.), and for cases before 1942, in the Reports of the United States Board of Tax Appeals (B.T.A.).

Memorandum decisions are related to established legal issues and, in contrast, hinge on interpretations of fact. They are decided by the trial judge. Memorandum Decisions (T.C. Memo.) are unofficially published in two competing publications, both titled Tax Court Memorandum Decisions (T.C.M.). Competing versions of the T.C.M. (with different pagination schemes) are published by RIA and CCH.

For more information on researching federal case law, please see the Law Library's research guides:

The Internal Revenue Service publishes several types of agency guidance that interpret or apply the IRC, Treasury regulations, and case law. They can also establish procedures.

The types of agency guidance are:

1. Revenue Rulings (Rev. Rul.): A revenue ruling is an official interpretation by the IRS of the Internal Revenue Code, related statutes, tax treaties and regulations. It is the conclusion of the IRS on how the law is applied to a specific set of facts.
2. Revenue Procedures (Rev. Proc.): A revenue procedure is an official statement of a procedure that affects the rights or duties of taxpayers or other members of the public under the Internal Revenue Code, related statutes, tax treaties and regulations and that should be a matter of public knowledge.
3. Notices: A notice is a public pronouncement that may contain guidance that involves substantive interpretations of the Internal Revenue Code or other provisions of the law.
4. Announcements: An announcement is a public pronouncement that has only immediate or short-term value.

Internal Revenue Service guidance is published weekly in the Internal Revenue Bulletin, and compiled annually in the Cumulative Bulletin.

The IRS website also provides digital copies of the Internal Revenue Bulletin from 1995 to the present: