Despite its vital role in the modern marketplace, attention has largely escaped the notice of the antitrust community. Existing discourse exhibits a variety of misconceptions and flawed prescriptions. One example is the widely held notion that attention markets necessitate two-sided platform analysis. Another is a marked overemphasis on data-collection practices. Yet another is that human users, viewers, and listeners are “the consumers” in these markets. And a fourth is that competition for attention occurs within one massive relevant market, obviating the possibility that any single firm could exercise market power. In light of nascent enforcement actions against Facebook, Google, and others, these defects require immediate correction. This Article begins by explaining the basic economics of attention markets, which often involve zero-price barter transactions. It turns next to the appropriate antitrust methodology for market definition. Attention markets need not encompass two “sides”; instead, they are best understood as traditional top-down distribution systems. The oft-used SSNIP test is facially unworkable in zero-price attention markets, but a SSNIC or SSNDQ variant may offer some utility. Practical indicia will often be more useful, however, given the unwieldy nature of hypothetical-monopolist tests generally. Regardless of methodology, courts and enforcers should take care to avoid the “massive market” fallacy espoused by some commentators. When assessing market power, both market shares and direct evidence can be useful. The three most common methods for assigning shares—time on-site, active users, and advertising revenues—each shed light on the issue, despite early judicial skepticism. That said, wherever available, direct evidence regarding attention-cost changes and competitive responses should play a primary role. As to anticompetitive effects, the “attention overcharge” should be a core concern. Recent enforcement efforts have employed alternative framing devices, but these will often be suboptimal. Finally, competition for attention can lead to overuse, overconsumption, heightened racial and gender animus, and other societal ills. In response, courts and enforcement authorities should extend leniency to certain attention-related conduct that might initially appear harmful.