The two main agencies involved in Tax Law Regulation are the U.S. Department of the Treasury and the Internal Revenue Service (IRS). The IRS is a bureau of the US Treasury Department. The IRS' website has a page titled "Understanding IRS Guidance - A Brief Primer" with information regarding regulations, revenue rulings, revenue procedures, private letter rulings, technical advice memorandums, notices, and announcements.
The Treasury Department and the IRS make proposed regulations (notice and public comment required), temporary regulations (effective upon publication, no notice and public comment required, expire in 3 years), and final regulations. Both temporary regulations and final regulations are published as Treasury Decisions (TD). Also, according to the Bluebook, regulations need not be cited to using their CFR citation (see below). They are cited to as Treasury Regulations, using a numbering scheme in the same way as the 1986 Internal Revenue Code, so that the sections are preceded by a number indicating the type of tax that the regulation is associated with (ex: Income Tax 1, Estate Tax 20, Gift Tax 25).
Title 26 of the Code of Federal Regulations (CFR) contains many tax law related regulations. The CFR is also available via Westlaw (with annotations), Lexis, Bloomberg Law, and eCFR (updated daily).
Title 26 - Internal Revenue, Chapter I - Internal Revenue Service, Department of the Treasury is broken down into a number of subchapters:
Internal Revenue Bulletins (IRB) - The IRB is the authoritative instrument that the IRS uses to announce official rulings and procedures and for publishing Treasury Decisions, Executive Orders, Tax Conventions, legislation, court decisions, and other items of general interest. They are also available in HTML and PDF. It contains final regulations, temporary regulations, and proposed regulations.
FederalRegister.gov - FederalRegister.gov contains proposed regulations, temporary regulations, and final regulations. Users can view documents by agency, such as the IRS or the Treasury Department. Users can also run keyword searches to find documents.
Bloomberg Law: Tax - Treasury regulations and proposed treasury regulations are available on Bloomberg Law: Tax, as well as many other tax law resources. Users can view citing cases and related content for regulations.
VitalLaw - VitalLaw has a database for federal tax regulations and notices of proposed rulemaking within its Tax - Federal Practice Area.
Lexis Tax - Lexis Tax has databases for final, temporary, and proposed regulations. Users can view citing materials by Shepardizing the regulations and view annotations that include notes and research research reference and practice aids.
Checkpoint - Checkpoint includes Federal Tax Coordinator 2d, US Tax Reporter, and other tax related resources. Final, temporary, proposed regulations and preambles are available. Users can view explanations, annotations, related Federal Tax Coordinator documents, related code sections, and related committee reports for regulations and proposed regulations.
Westlaw- Westlaw has treasury regulations available with annotations and a searchable database of tax proposed & adopted regulations.
The IRS publishes a number of guidance documents in the Internal Revenue Bulletins (IRB). The IRB is the authoritative instrument that the IRS uses to announce official rulings and procedures and for publishing Treasury Decisions, Executive Orders, Tax Conventions, legislation, court decisions, and other items of general interest. It contains final regulations, temporary regulations, and proposed regulations. The IRS used to publish the Cumulative Bulletin (CB) twice annually (1919-2008).
Revenue Rulings - 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.
Revenue Procedure - 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.
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.
Actions on Decisions (acquiescence and nonacquiescence) - An Action on Decision (AOD) is a formal memorandum prepared by the IRS Office of Chief Counsel that announces the future litigation position the IRS will take with regard to the court decision addressed by the AOD. AODs are available on IRS.gov.
Announcements - An announcement is a public pronouncement that has only immediate or short-term value.
Bloomberg Law: Tax - Bloomberg Law has a section for IRS Agency Documents with some specific document types listed. To view all, select All IRS Agency Documents. The documents are broken down by type.
VitalLaw - VitalLaw has a practice area for Tax, which includes IRS guidance documents.
RIA Checkpoint - RIA has a database for IRS Rulings and Releases that includes IRS guidance documents.
Lexis Tax - Lexis Tax has a section for IRS Rulings and Releases that includes IRS guidance documents.
Westlaw - Westlaw has a database of IRS materials that includes IRS guidance documents.
HeinOnline - HeinOnline has historical versions of the Cumulative Bulletin (1919-2008), as well as newer versions of the IRB (2009-2018).
The IRS creates many other guidance documents and publications. Below is a non-exhaustive list. For additional information, visit IRS.gov.
Letter Rulings and Private Letter Rulings (PLR) - A private letter ruling, or PLR, is a written statement issued to a taxpayer that interprets and applies tax laws to the taxpayer's specific set of facts. A PLR is issued to establish with certainty the federal tax consequences of a particular transaction before the transaction is consummated or before the taxpayer's return is filed.
Determination letters - responses from district office, only based on well-established rules that apply to the issue.
Technical Advice Memoranda (TAM) - A technical advice memorandum, or TAM, is guidance furnished by the Office of Chief Counsel upon the request of an IRS director or an area director, appeals, in response to technical or procedural questions that develop during a proceeding. TAMs and other IRS Written Determinations are available on IRS.gov.
General Counsel Memoranda - include reasoning and authority used in revenue rulings, private letter rulings, and technical advice memoranda.
Field Service Advice (FSA) - Field Service Advice are documents prepared by Field attorneys in the Office of Chief Counsel that are reviewed by an Associate Office, and subsequently issued to field or service center campus employees of the Internal Revenue Service. They are available on IRS.gov
Information Letters - An information letter provides general statements of well-defined law without applying them to a specific set of facts. They are furnished by the IRS National Office in response to requests for general information by taxpayers, by congress-persons on behalf of constituents, or by congress-persons on their own behalf. Information Letters are available on IRS.gov.
Internal Revenue Manual (IRM) - The Internal Revenue Manual contains policies of the IRS. The IRM is available on IRS.gov.
Bloomberg Law: Tax - Bloomberg Law has a section for IRS Agency Documents with some specific document types listed. To view all, select All IRS Agency Documents. The documents are broken down by type.
VitalLaw - VitalLaw has a practice area for Tax, which includes IRS guidance documents.
Checkpoint - Checkpoint has a database for IRS Rulings and Releases that includes IRS guidance documents.
Lexis Tax - Lexis Tax has a section for IRS Rulings and Releases that includes IRS guidance documents.
Westlaw - Westlaw Edge has a database of IRS materials that includes IRS guidance documents.
