Merger review in the United States runs through the Hart-Scott-Rodino premerger notification program, in force since 1978: deals above size thresholds — 126.4 million dollars in 2025's adjusted valuation test — must be filed with both the Federal Trade Commission and the Justice Department's Antitrust Division before closing, paying a filing fee and waiting out a 30-day initial waiting period. The agencies received roughly 2,000 to 3,000 HSR filings a year in the 2020s, challenged a small fraction, and the rest cleared. The reviewing framework is the Clayton Act's Section 7 — acquisitions whose effect may be substantially to lessen competition — plus the 2023 merger guidelines the agencies issued jointly, and the location of the fight is usually one of two courts: the parties' federal district court or the FTC's in-house tribunal. Reliable News publishes information, not legal advice; this explainer maps the machinery.
How does the 30-day clock work?
Both parties file HSR forms describing the deal, ownership, and overlaps; the initial waiting period is 30 days — 15 for cash tender offers. The FTC's Premerger Notification Office designates one agency to review — the FTC taking retail, pharma, tech platforms, and consumer goods, the DOJ taking transportation, media, telecom, banking, and industrials by rough division. In the vast majority of filings the period expires and the deal closes. Where questions arise, the reviewing agency issues a second request — a demand for documents and data that extends the waiting period, typically by months to a year, and signals a serious investigation. In fiscal 2024, second requests ran to roughly 30 to 40 deals of thousands filed, per agency statistics.
What are the agencies looking for?
Market concentration and competitive harm. The analytical sequence under the 2023 guidelines: define relevant markets, measure concentration with the HHI index, and assess whether the merger creates or enhances market power — through coordinated conduct, unilateral effects, or eliminating a maverick. The 2023 guidelines added structural presumptions for high-concentration deals and revived theories — potential competition, serial acquisitions, platforms entangling rivals — that had faded in the 1980s-2000s consumer-welfare era. Efficiency defenses exist but carry a heavy burden: cognizable, merger-specific, and verified. Labor-market harm, the 2023 guidelines' most novel theme, treats worker-side monopsony as cognizable injury.
What can the agencies actually do?
Three outcomes. Clear: let the waiting period lapse or grant early termination. Negotiate: consent decrees or settlement agreements divesting assets or restricting conduct — the standard remedy for most challenged deals, the airlines' slot divestitures being the classic form. Or sue: the DOJ in federal district court for an injunction; the FTC either in district court or before its administrative law judge under Section 5 of the FTC Act — the in-house route the Supreme Court constrained in 2024's SEC v. Jarkesy decision, holding that civil-penalty-style claims belong before juries when they rest on fraud-like theories, a ruling that pushed the FTC toward federal court for merger remedies. Losers in agency court can appeal; parties facing injunctions abandon deals — the record of 2023-2025 includes abandoned megadeals in groceries and media.
What changed in the 2023 guidelines?
The 2023 joint guidelines replaced the 2010 horizontal and vertical guidance with a more enforcement-friendly framework: lower structural presumptions from the 1982-84 lineage, market-share triggers for dominant firms' acquisitions, attention to serial roll-ups, and the labor-market theory. Courts are not bound by them — the agencies must still prove cases under the Clayton Act and case law — and the early record was mixed: the agencies won retail-grocery and airline-coalition cases and lost others where courts found the markets broader than pleaded. The guidelines bind the agencies' own screening, which is enough to change deal flow: HSR filings and deal announcements shift as counsel prices the enforcement posture.
What about foreign review?
A separate layer: the EU's merger regulation, Britain's CMA, China's SAMR, and other regimes each require notification and can condition or block the same deal. Global deals now run a gauntlet — China's conditional approvals with hold-separate remedies became a standard feature of U.S.-origin deals in the 2010s-2020s, and CMA blockades, as in Microsoft-Activision's initial UK refusal before restructuring, changed deal structures worldwide. U.S. agencies coordinate with counterparts through the International Competition Network, but decisions are sovereign.
Who polices closed deals?
Anyone, later: the Clayton Act allows challenges after closing — the FTC's post-consummation cases against consumuated tech acquisitions, and private plaintiffs can sue for injunctions and treble damages for antitrust injury. The analysis: the HSR system is disclosure-first — the public learns what is being bought, and the fight is over proof of harm in markets that are themselves contested constructs, which is why the same merger can look lawful in one court's market definition and unlawful in another's; the 2023 guidelines moved the agencies' prior, and Jarkesy moved the forum, leaving the regime's center — agency screening power over every large deal — untouched. What would change the reading is Congress amending HSR or the Clayton Act, which the 2024-2025 legislative sessions considered without enacting.
Frequently asked questions
What is Hart-Scott-Rodino review?
The premerger notification regime: deals above the 126.4-million-dollar 2025 threshold must be filed with the FTC and DOJ, which have a 30-day waiting period to investigate before closing. Second requests extend the clock for the small fraction of deals deeply probed.
Do the FTC and DOJ split merger review?
Yes, by informal industry division: the FTC takes retail, pharma, platforms, consumer goods; the DOJ takes transport, media, banking, industrials. Either can sue to block — DOJ in federal court, FTC in court or before its own tribunal.
What is a second request?
A demand for extensive documents, data, and depositions that extends the HSR waiting period, usually by six months to a year. It signals a serious investigation; most second-request deals end in abandonment, divestiture settlement, or litigation.
Can a closed merger still be challenged?
Yes. The Clayton Act permits post-closing challenges by the agencies or private plaintiffs, and the FTC has unwound consummated acquisitions. Parties to review may also face EU, UK, or Chinese conditions separately.
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