Antitrust analysis of online markets is a hot topic around the world. In a number of jurisdictions, online markets already have been subject to antitrust review in merger or conduct cases. In other jurisdictions, these issues are in a nascent stage of policy. A number of lessons can be learned from the cases to date involving online markets with regard to optimal antitrust policy. What these cases tend to share are some basic features as to how online markets work. Some jurisdictions understand the particular dynamics of multi-sided online markets. Other competition authorities sometimes may misidentify these markets. This essay outlines five areas in which online markets may be different from traditional markets for antitrust purposes. The essay also explores why such markets require a more careful consideration from antitrust authorities and courts in their respective antitrust analyses.