Payment For Order Flow

When you tap "buy" in a commission-free trading app, your order very often doesn't go to an exchange at all. It's routed to a small number of wholesale market makers — firms like Citadel Securities or Virtu — who fill it themselves, out of their own inventory, and pay your broker a small fee for the right to see and fill that order. That fee is payment for order flow, and it's a large part of how "free" retail trading is actually paid for.

Why a wholesaler is willing to pay for your order

A wholesaler wants your order specifically because retail orders are, on average, less likely to be trading on short-term information than institutional or professional orders. A retail investor buying 100 shares is statistically much less likely to know something the market doesn't than a hedge fund selling 500,000 shares in a hurry. Filling "uninformed" flow is safer and more profitable for a market maker than filling flow that might be trading against them, so wholesalers compete to buy access to it — and part of that competition takes the form of a direct cash payment to the broker who controls the order.

Walking through one order

Say you place a market buy for 100 shares of a stock quoted 50.00 / 50.02 (a 2-cent spread) on the public NBBO. Your broker routes the order to a wholesaler instead of an exchange. The wholesaler:

• Fills you at 50.015 — half a cent of price improvement versus the 50.02 you'd have paid crossing the public spread, which shows up as a real, quantifiable benefit to you.
• Pockets the other half-cent of spread as its own profit, since it filled you from its own inventory rather than paying an exchange to trade.
• Pays your broker roughly $0.001–$0.002 per share (figures vary by stock and broker) for having routed the order there rather than to an exchange or another wholesaler.

Party	What happens
You	Bought at 50.015 — better than the 50.02 public offer
Wholesaler	Earns the remaining half-cent of spread, pays the broker a routing fee
Broker	Receives the routing fee, funds "commission-free" trading
Public exchange	Never sees this order at all

The controversy in one sentence

Critics argue PFOF creates an incentive for brokers to route to whichever wholesaler pays the most, not necessarily whichever wholesaler gives clients the best execution, even though the two often align in practice because competing for order flow also pushes wholesalers to offer more price improvement. Defenders point to measured execution quality data showing retail investors frequently do get meaningfully better prices than the public NBBO under this system — see Measuring Retail Execution Quality — and argue that without PFOF-funded routing, commission-free trading itself might not exist. Retail Liquidity Programs And Price Improvement covers exchange-side mechanisms built to compete for the same flow without payment changing hands, and Maker-Taker Pricing And Market Quality covers the analogous, longer-running debate over rebates paid to liquidity providers on lit exchanges.

Why it's largely an options and equities phenomenon

PFOF is most visible in US equities and listed options, and it exists at the scale it does partly because of Reg NMS's carve-out allowing off-exchange execution as long as the price given is at least as good as the protected NBBO — see Regulation NMS. Options markets in particular route an especially large share of retail flow this way, because options quotes are wide relative to the underlying stock and the population of retail options traders skews toward smaller, less price-sensitive orders that wholesalers are especially eager to internalize. Regulators in some other jurisdictions — the UK and EU under MiFID II among them — have gone further than the US and banned the practice outright, arguing the underlying conflict of interest can't be fully resolved just through disclosure, which is a genuine and unresolved policy disagreement rather than a settled question.

For a trading firm, the economics are straightforward even if the ethics are debated: PFOF revenue is one of a small number of ways a "commission-free" broker actually makes money on order flow, alongside interest earned on uninvested client cash and payment for securities lending, and any serious comparison of brokers has to weigh all of these together rather than treat "zero commission" as meaning free.