Principal media is advertising inventory that a media agency buys with its own money, or commits to buying, and then resells to its clients at a price it does not have to disclose. The agency stops acting as the advertiser's agent, spending client money at cost plus a disclosed fee, and becomes the owner, or "principal", of the media it sells. The Association of National Advertisers (ANA) defines it as "the practice in which agencies purchase media inventory themselves and resell it to clients, rather than acting solely as agents." It serves three pressures at once: advertisers want lower prices, agencies want margin that fees no longer supply, and publishers want guaranteed revenue.

The term has regional variants. The Incorporated Society of British Advertisers (ISBA) prefers proprietary media, and also lists "inventory media" and "principal based media" as synonyms. Inside WPP, according to documents made public in a 2026 court case, the same revenue line was described as "proprietary media trading" and "purchase risk media deals".

How a principal buy works

In a conventional agent-based buy, the agency negotiates with a publisher, signs an insertion order in the client's name, passes through the vendor invoice and charges a fee. The client can audit what the publisher was paid.

A principal buy breaks that chain into two separate sales. First, the agency or a trading unit of its holding company negotiates with a seller, often in bulk and in advance: a block of TV spots in an upfront, a tranche of connected TV (CTV) impressions, a guaranteed volume on a platform. Second, it sells slices of that inventory to individual clients at a single all-in price per unit, typically a cost per thousand impressions (CPM). The difference between the two prices is the agency's spread, a margin that neither counterparty sees on both sides.

According to the ANA's March 2026 report, "marketers do not have access to vendor invoices, so they do not know the price the agency paid (if any) to acquire the media." Audits can still check delivery - impressions, viewability, invalid traffic - and what the agency charged, but not the acquisition cost. For digital buys, the ANA notes that most principal deals also require advertisers to accept that technology, platform and data costs are bundled into the unit price. Rebates that would otherwise flow back to the client under an agent contract are typically forfeited.

In theory the agency carries the inventory risk: if it cannot resell what it bought, it absorbs the loss. Whether that risk is real is disputed. ADLINGO, an industry glossary, notes it is "increasingly debatable" whether agencies actually purchase inventory in advance rather than simply reselling on demand. Because the margin is invoiced as media, it also inflates the share of a budget that appears to be working media.

On the sell side, the counterparties are broadcasters, streaming services, publishers and platforms willing to trade price for committed volume. On the buy side, the operators are holding company trading units. Stagwell runs its own through Stagwell Owned Media, which on October 26, 2026 added Freestar, a supply partner to more than 1,000 publishers, according to ADOTAT.

From space brokers to trading desks

Buying space and reselling it is how the advertising agency began. Forrester traces principal-style trading back to N. W. Ayer and J. Walter Thompson, nineteenth-century firms that bought newspaper space wholesale and sold it on to advertisers. The fiduciary agency model, in which the agency spends client money transparently for a fee, displaced that arrangement over the twentieth century.

The modern version re-emerged with programmatic buying and global consolidation. The decisive moment was June 7, 2016, when the ANA published a K2 Intelligence investigation into US media transparency. Based on 143 interviews with 150 sources, it found rebates worth between 1.67% and 20% of aggregate media spend, and described principal markups of 30% to 90%. According to Forrester, principal trading "fell out of favor" after that report as marketers tightened contracts.

It returned under a new name. In June 2023 the ANA's media agency contract template, version 3.0, replaced its principal and inventory mark-up provisions with a broader section on non-transparent services. In May 2024 the ANA published "The Acceleration of Principal Media", based on a survey fielded from February 1 to March 1, 2024, which found that 47% of marketers had used it in the previous 12 months. ISBA issued its own briefing on June 12, 2025 and fuller guidance, "Proprietary Media: Transparency, Governance and Effectiveness", on February 18, 2026.

Why it matters to advertisers

The ANA's 2026 study, published March 19, 2026 and based on 114 marketers surveyed between October 10 and December 19, 2025, found that 58% had used principal media in the past year, up from 47%. Some 56% expected to use it in the year ahead, against 41% two years earlier, and 22% now put more than a quarter of their media budget through it. Television dominates: 74% of users buy TV this way, while open web digital fell from 68% to 43%.

Forrester predicted in January 2026 that principal media would reach one-third of total media billings during the year. Hard revenue figures remain scarce. A 2024 internal memo made public through WPP's counter-filing in the lawsuit brought by former GroupM executive Richard Foster put GroupM's "non-product related income" above $1 billion in 2023, with 15% growth forecast. PPC Land's WPP profile describes the practice there as "legal, contested and rarely quantified". AdExchanger estimated Omnicom's principal revenue for 2025 at more than $2 billion, an inference from a $4.1 billion line of third-party service costs rather than a disclosed figure.

The Omnicom acquisition of Interpublic, completed in late 2025, matters here because principal trading rewards scale. The ANA report says the merger "effectively doubled" Omnicom's buying power. Omnicom's third-party service costs reached almost $2.9 billion in the first half of 2026, up from $1.7 billion a year earlier, a line PPC Land tracked in its WPP results coverage.

Agencies argue that clients benefit. The ANA's own respondents ranked reduced cost as the top advantage (76%), and 54% cited access to exclusive or higher-quality inventory. Not every holding company leans in equally. Publicis chief executive Arthur Sadoun said of principal media, according to AdExchanger, "It is not a strategic priority for us."

Where the model is contested

The central objection is a conflict of interest. An agency that owns inventory has a reason to recommend it whether or not it fits the plan. In the ANA's 2026 survey, 90% of marketers cited uncertainty about whether recommendations were in their best interest, up from 79%. Loss of audit rights (77%) and lack of visibility into agency profit (76%) followed. Nick Manning, a UK media consultant, told ADOTAT in 2026 that "self-preference is inevitable" when agencies buy and sell on their own account.

Governance has not kept pace. The share of marketers with formal guidelines fell from 62% to 57%. In the March report, 63% said agency contracts addressed principal media; a separate ANA survey in June 2026 found 61% had contracts that specifically covered it. The samples differ, but both imply roughly four in ten advertisers lack explicit terms.

Quality is a second concern. One consumer goods executive in the ANA study warned that cheaper media is sometimes "less costly for a reason", and a case study described resold inventory that failed to meet quality expectations. A third concern is that the discounts themselves cannot be verified, since the benchmark - what the agency paid - is withheld by design. The ANA recommends per-purchase written approval, a ban on mixing principal and agent buys, budget caps, quarterly reporting and pitch price guarantees based only on agent transactions. On November 2, 2026 an ANA magazine op-ed by Rick Corteville gave principal media "a failing grade".

Sources also disagree on what the model is. The ANA and ISBA treat any undisclosed resale margin as principal media; Forrester frames it as buying "at an undisclosed discount" and reselling "with a price markup"; ADLINGO questions whether advance purchase still happens at all.

What principal media is not

Agency rebates and volume bonuses. Rebates are payments from a seller back to an agency based on spend. Under an agent contract they belong to the client. Principal media replaces the rebate with a resale margin, which is why the ANA warns rebates are usually forfeited.

Barter. Barter firms such as Active International exchange an advertiser's surplus goods for media credits. The trade is disclosed as a barter transaction, not a resale of agency-owned inventory.

Joint business plans. A JBP is a multi-year spend commitment in return for vendor discounts. It can create the volume that makes principal buying viable, but it is a plan, not a trading model.

Principal trading in finance. In securities markets a broker trading as principal also sells from its own book, but regulators impose disclosure and best-execution duties. No equivalent rule applies to media.

Recent developments

Digiday reported on July 27, 2026 that a holding company offered to cover a chief marketing officer's entire AI infrastructure bill on condition that 70% of the media budget ran through principal inventory. By August, agencies were turning AI tokens into a margin business, with free tokens offered to clients who signed principal buying commitments. The ANA extended the critique to creator budgets in August 2026, comparing bundled talent and agency fees to the undisclosed principal markup. PPC Land has since described principal-based buying as a form of backward vertical integration by agencies. As of November 2026, no regulator has acted against the practice; the constraints remain contractual.

Timeline

  • 19th century - N. W. Ayer and J. Walter Thompson buy newspaper space wholesale and resell it, the model Forrester identifies as principal media's origin.
  • June 7, 2016 - ANA publishes the K2 Intelligence transparency report: rebates of 1.67% to 20% of spend, principal markups of 30% to 90%.
  • June 6, 2023 - ANA media agency contract template 3.0 replaces principal and inventory mark-up clauses with a non-transparent services section.
  • February 1 to March 1, 2024 - ANA fields its first principal media survey.
  • May 2024 - ANA publishes "The Acceleration of Principal Media"; 47% of marketers report use.
  • June 12, 2025 - ISBA publishes "The Ins and outs of Inventory/Proprietary Media".
  • November 2025 - Richard Foster files a whistleblower suit against WPP; WPP's counter-filing later exposes GroupM trading income above $1 billion.
  • Late 2025 - Omnicom completes its acquisition of Interpublic.
  • January 15, 2026 - Forrester predicts principal media will reach one-third of media billings in 2026.
  • February 18, 2026 - ISBA launches "Proprietary Media: Transparency, Governance and Effectiveness".
  • March 19, 2026 - ANA report finds 58% usage and 90% concern over best interest.
  • June 2026 - ANA survey finds 61% of contracts specifically address principal media.
  • July 27, 2026 - Digiday reports AI infrastructure bills offered against 70% principal commitments.
  • October 26, 2026 - Stagwell adds Freestar to Stagwell Owned Media.
  • November 2, 2026 - ANA op-ed gives principal media "a failing grade".

Summary

Who: Media agencies and holding company trading units (WPP, Omnicom, Publicis, Stagwell and others) buy and resell inventory; advertisers buy it; publishers, broadcasters and platforms supply it; the ANA, ISBA and WFA set governance guidance.

What: A trading model in which the agency owns the media it sells, charging clients a single unit price and keeping an undisclosed margin instead of passing through vendor costs for a fee.

When: Rooted in nineteenth-century space brokering, curbed after the ANA's June 2016 transparency report, and growing again since the ANA's May 2024 study; 58% of surveyed marketers used it by early 2026.

Where: Most heavily in television (74% of users) and CTV, and less in open web digital (43%), across the US, UK and global markets where holding companies trade at scale.

Why: Advertisers seek lower prices, agencies seek margin as fees fall, and publishers seek guaranteed revenue; the cost is lost transparency, weaker audit rights and a conflict of interest that 90% of marketers in the ANA survey now cite.