A preliminary injunction is a court order that stops a party from doing something, or occasionally compels it to act, while the lawsuit over that conduct is still being decided. It is provisional relief, not a verdict, and it exists because litigation runs for years while some harm cannot be repaired by a payment at the end: a market position lost, a data set copied, a compliance deadline already passed. Courts call it an extraordinary remedy never awarded as of right, and the burden sits entirely on the movant.

The four factors

The controlling United States standard comes from Winter v. Natural Resources Defense Council, decided on November 12, 2008. A movant must establish that it is likely to succeed on the merits, that it is likely to suffer irreparable harm without an injunction, that the balance of equities tips in its favour, and that an injunction serves the public interest.

The second factor carries most of the weight in practice. Winter rejected the looser approach under which a strong showing on the merits let a court grant relief on a mere possibility of harm. Irreparable means harm that money cannot cure: loss of goodwill, disclosure of a trade secret, the erosion of a competitive structure. A quantifiable revenue shortfall is generally not irreparable, because damages can fix it later.

Circuits differ on how rigidly the four must be proved. The Ninth Circuit held in Alliance for the Wild Rockies v. Cottrell in 2011 that a sliding scale survived Winter, so serious questions can suffice where the balance of hardships tips sharply. Other circuits treat each factor as an independent gate.

State practice diverges further, and shifts again when the plaintiff is a government. In the price-fixing case brought against Amazon, California argued that once the state shows a reasonable probability of prevailing, a rebuttable presumption arises that public harm outweighs harm to the defendant, one it said Amazon could not rebut because the conduct was already alleged to be unlawful.

Rule 65 mechanics

Federal procedure sits in Rule 65 of the Federal Rules of Civil Procedure. Subsection (a) requires notice to the adverse party and lets the court consolidate the hearing with a trial on the merits. Subsection (b) governs the faster, cruder instrument: a temporary restraining order, which can issue without notice only where a sworn affidavit or verified complaint shows immediate and irreparable injury before the other side can be heard. Such an order must state the date and hour of issuance, describe the injury, explain why notice was not required, and expires within 14 days unless extended for good cause or by consent.

Subsection (c) is the one commercial parties negotiate hardest. A court may issue either order only if the movant gives security covering costs and damages should the restraint prove wrongful, though the United States and its agencies are exempt. The amount is discretionary and the spread is wide. Perplexity asked for a bond of one billion dollars when Amazon won its order, and the district court refused. In the dispute between Automattic and WPEngine, the court required no bond at all, reasoning that an order returning matters to the arrangements of September 20, 2024 was unlikely to harm the enjoined party.

Subsection (d) requires every injunction to state its reasons and terms specifically, describing the restrained acts in reasonable detail rather than by cross-reference. It binds the parties, their officers and agents, and anyone in active concert with them who receives actual notice. Short administrative stays are common at issuance: both the Perplexity order and the one restraining the Department of War over its Anthropic supply chain designation carried seven days.

Origin and evolution

The instrument descends from the English Court of Chancery, where the interlocutory injunction developed as equity's answer to the common law's inability to act before final judgment. English practice was consolidated in American Cyanamid v. Ethicon in 1975, which set a serious-question test paired with the adequacy of damages and the balance of convenience. American federal practice was codified with the Federal Rules in 1938, and the restraining order limit stood at 10 days until the December 1, 2009 time-computation amendments lengthened it to 14.

Two Supreme Court decisions then tightened the substance. eBay Inc. v. MercExchange in 2006 ended the near-automatic grant of permanent injunctions in patent cases, installing a four-factor test; Winter followed in 2008. Lower courts extended the logic: the Ninth Circuit in Herb Reed Enterprises v. Florida Entertainment Management in 2013 and the Third Circuit in Ferring Pharmaceuticals v. Watson Pharmaceuticals on August 26, 2014 scrapped the presumption that trademark and false advertising plaintiffs suffer irreparable harm automatically. Advertisers pursuing competitor claims must now prove that harm with evidence.

The most recent structural change concerns reach rather than standard. In Trump v. CASA, decided June 27, 2025, a six-justice majority held that the Judiciary Act of 1789 does not authorise universal injunctions extending beyond what is needed to give complete relief to the parties before the court. Challenges to federal policy have since shifted toward class certification.

Why it matters for the marketing community

Preliminary relief has become the battleground in disputes over how advertising technology functions, because the cases outlast the products.

Agent traffic is the clearest instance. A federal court barred Perplexity's Comet browser from using artificial intelligence agents inside password-protected Amazon accounts on March 9, 2026, finding Amazon likely to succeed under the Computer Fraud and Abuse Act. Advertising revenue was the commercial subtext, since an assistant navigating a retail site can bypass the sponsored placements that generate it. Publisher groups intervened for Amazon, arguing that agents presenting as human traffic corrupt advertising metrics.

Measurement contracts have proved equally susceptible. A judge in the Southern District of New York restrained Nielsen on December 30, 2025 from enforcing a policy that tied national radio ratings to purchases of local products, finding Cumulus Media likely to prevail under Section 2 of the Sherman Act and writing a presumptive pricing benchmark into the order.

State statutes governing personalised advertising are litigated the same way, before effective dates rather than after. Platforms failed to enjoin provisions of California Senate Bill 976 on August 5, 2026, leaving an age assurance deadline of January 1, 2027 intact, and xAI was refused an order blocking the state's artificial intelligence training data disclosure law. Both denials followed a partial injunction against Senate Bill 976 that left most of the statute enforceable.

Limitations and disputes

The record is thin by design. Judges rule on affidavits and limited discovery, long before the evidence is complete, which makes reversal a live possibility. The Perplexity order was vacated on appeal after the presiding judge had voiced doubt at the hearing about whether a 1984 anti-hacking statute should reach the conduct at all.

Factor-collapsing is a recurring complaint. Perplexity's opening brief argued that the district court had folded all four requirements into one finding, inferring irreparable harm from likely success rather than analysing it separately. Appellate courts sometimes agree, and sometimes simply stay the order while they decide, which is what Nielsen obtained from the Second Circuit.

Cost and asymmetry attract criticism. Emergency briefing is expensive, favouring well-resourced litigants, and a bond can be waived or set low, leaving a wrongly enjoined defendant with little recourse. Critics of the opposite tendency note that a denied injunction can moot a case in practice, because conduct completed during litigation is rarely undone.

Not the same as

A temporary restraining order is shorter, faster and frequently granted without hearing the other side, functioning as a bridge to the injunction hearing. The sequence is visible in the Federal Trade Commission action against an e-commerce scheme, where a restraining order in March 2025 halted operations before preliminary injunctions and a receiver arrived in April.

A permanent injunction issues after a final ruling on liability and needs no prediction about the merits. The behavioural remedies imposed when Judge Leonie Brinkema declined to break up Google's ad exchange on September 2, 2026 are permanent relief, as is the Google Play order the Ninth Circuit refused to stay in September 2025.

A stay pending appeal suspends an order already entered rather than restraining private conduct. Its factors resemble the injunction test, but the question is preservation of the appellate court's jurisdiction.

Interim measures are the European analogue, imposed by a competition authority rather than a court. Brussels ordered Meta on June 9, 2026 to restore third-party access to the WhatsApp Business API, only the second use of the power under Article 8 of Regulation 1/2003 since 2019. France used the same instrument against Google in April 2020 over publisher licensing, and industry groups have pressed for the Article 24 equivalent under the Digital Markets Act. German courts run a parallel civil track, the einstweilige Verfuegung under sections 935 and 940 of the Zivilprozessordnung, which plastic surgeons invoked unsuccessfully against Google AI Overviews in Frankfurt on September 10, 2025.

Recent developments

The Ninth Circuit vacated the Amazon order on August 4, 2026 and remanded, holding that the user rather than the developer accesses a website through an agent. None of the four factors favoured Amazon, the panel found, treating a degraded shopping experience as too abstract to count as irreparable harm. It declined to set any legal regime for autonomous software, leaving terms of service and technical identification as the remaining controls.

In the state antitrust case against Amazon, the hearing on California's injunction motion was set for July 23, 2026, ahead of a trial scheduled for January 19, 2027.

Timeline

  • 1938: The Federal Rules of Civil Procedure are adopted, codifying injunctive practice in Rule 65
  • 1975: The House of Lords decides American Cyanamid v. Ethicon, setting the English interlocutory test
  • 2003: Regulation 1/2003 gives the European Commission power to impose interim measures under Article 8
  • 2006: eBay Inc. v. MercExchange installs a four-factor test for permanent injunctions
  • November 12, 2008: Winter v. Natural Resources Defense Council rejects the possibility standard for irreparable harm
  • December 1, 2009: Amendments extend the temporary restraining order limit from 10 days to 14
  • 2011: Alliance for the Wild Rockies v. Cottrell preserves a sliding-scale approach in the Ninth Circuit
  • 2013 to 2014: Herb Reed and Ferring end the presumption of irreparable harm in trademark and false advertising cases
  • 2019: The European Commission uses Regulation 1/2003 interim measures against Broadcom, the first such order in decades
  • April 2020: The French competition authority imposes interim measures on Google over publisher licensing
  • December 2024: A federal court orders Automattic to restore WPEngine's access without requiring a bond
  • December 31, 2024: A partial injunction against California Senate Bill 976 leaves most provisions enforceable
  • June 27, 2025: Trump v. CASA restricts universal injunctions to relief needed by the named parties
  • December 30, 2025: A New York court restrains Nielsen's radio ratings policy
  • March 9, 2026: A California court bars Perplexity's Comet assistant from Amazon accounts
  • June 9, 2026: The European Commission orders Meta to reopen the WhatsApp Business API to rival assistants
  • August 4, 2026: The Ninth Circuit vacates the Amazon order and remands
  • August 5, 2026: A court denies injunctions against California Senate Bill 976 feed provisions

Summary

Who: Judges in trial courts, acting on a motion by a plaintiff or, less often, a counterclaiming defendant. In the advertising sector the movants have included Amazon, Cumulus Media, WPEngine, xAI, Meta, Google, state attorneys general and federal agencies.

What: A provisional court order restraining or compelling conduct until a case is decided, granted in United States federal practice on a showing of likely success on the merits, likely irreparable harm, a favourable balance of equities and consistency with the public interest, under Rule 65 of the Federal Rules of Civil Procedure.

When: Codified federally in 1938, tightened by eBay in 2006 and Winter in 2008, extended to 14-day restraining orders in 2009, and narrowed in scope by Trump v. CASA on June 27, 2025.

Where: United States federal and state trial courts, with functional equivalents in English interlocutory practice, the German einstweilige Verfuegung, and interim measures imposed by the European Commission and national competition authorities.

Why: The order determines what a company may do for the years a case takes to resolve, which in fast-moving advertising and artificial intelligence markets frequently settles the commercial question before any final judgment is reached.