Scope

The FASB Accounting Standards Codification (the "Codification" or "ASC") is the single source of authoritative US GAAP for all non-governmental entities, effective for reporting periods after September 15, 2009. Before the Codification, US GAAP was a layered hierarchy of FASB Statements, Interpretations, Technical Bulletins, EITF abstracts, AICPA pronouncements, and SEC guidance. The Codification consolidates all of that into a single research database, organized by Topic, Subtopic, Section, and Paragraph.

For governmental entities (state and local governments), the analogous authoritative source is the GASB Codification, not the FASB ASC. For private companies, the same FASB ASC applies, with limited Private Company Council (PCC) alternatives available for goodwill, certain intangibles, VIE consolidation, and interest rate swaps.

Structure

The Codification is hierarchical. A typical citation reads ASC 606-10-25-1, decoded as:

Topic — 606 (Revenue from Contracts with Customers). Topics are numbered 105–999 across nine areas (General Principles, Presentation, Assets, Liabilities, Equity, Revenue, Expenses, Broad Transactions, Industry).
Subtopic — 10 (Overall). Subtopic 10 is the general subtopic for almost every Topic. Other subtopics carve out specific transactions or sub-populations.
Section — 25 (Recognition). Sections follow a standardized numbering convention: 05 Overview, 10 Objectives, 15 Scope, 20 Glossary, 25 Recognition, 30 Initial Measurement, 35 Subsequent Measurement, 40 Derecognition, 45 Other Presentation, 50 Disclosure, 55 Implementation Guidance, 60 Relationships, 65 Transition, 70 Grandfathered Guidance, 75 XBRL Elements.
Paragraph — 1 (the first paragraph in that Section).

Once you internalize this structure, citations become navigable rather than opaque. "I need the disclosure requirements for leases" maps directly to ASC 842-XX-50, where XX is the relevant subtopic (20 for lessees, 30 for lessors).

How GAAP is updated

FASB amends the Codification through Accounting Standards Updates (ASUs). An ASU is the vehicle for the change; the Codification itself is the destination. Once an ASU is issued, the changes are integrated into the Codification with effective dates that vary by entity type (public business entities vs. all other entities) and by reporting period.

Notable recent ASUs to be aware of:

ASU 2014-09 — created ASC 606 (Revenue Recognition)
ASU 2016-02 — created ASC 842 (Leases)
ASU 2016-13 — created ASC 326 (Credit Losses / CECL)
ASU 2023-07 — expanded segment disclosure under ASC 280
ASU 2023-09 — expanded income tax disclosure under ASC 740

Hierarchy with SEC guidance

For SEC registrants, the Codification incorporates SEC content as a separate sub-layer within each Topic, labeled "S" sections (S99 for SEC materials, S25 for SEC recognition, etc.). This includes Staff Accounting Bulletins (SABs), the SEC Financial Reporting Codification, and certain SEC rules and interpretive releases. Non-SEC registrants ignore the S sections. SEC registrants must comply with both the FASB content and the S sections.

Common pitfalls

Citing pre-Codification literature. SFAS 13 doesn't exist anymore. Cite ASC 840 (now superseded by ASC 842) or ASC 842 directly. Pre-Codification citations in a current-period workpaper signal that the preparer hasn't refreshed their references — common and avoidable.
Missing the subtopic. "ASC 606" is incomplete. "ASC 606-10" or, more usefully, "ASC 606-10-25-1" is the actual citation. Memos that cite topics without subtopics or paragraphs are working at a level of abstraction that doesn't get you to a defensible position.
Confusing the ASU with the Codification. ASU 2014-09 is the document FASB issued. ASC 606 is where the rules now live. After the ASU's effective date, cite the Codification, not the ASU.

Access

FASB offers two access tiers. The Basic View is free with registration and provides the full text of the Codification. The Professional View is a paid subscription that adds cross-reference tables, archived versions, and advanced search. For most preparer-level research, the Basic View is sufficient.

For the full Codification, see asc.fasb.org.

Related references

IFRS Foundation (the international counterpart)
SEC Reporting (the layer that applies to public registrants)
All ASC-specific references on this site cite back here.