Online platforms support so many of our daily activities that we have become dependent on them in our personal and professional lives. We rely on them to buy and sell goods and services, to find information online and to keep in touch with each other. We use them for entertainment, news, transportation, accommodation, finding jobs and employees, finding apps and for many other purposes. Online platforms have also raised new and important policy questions, but the businesses themselves can be more complex than they appear so they are not always well understood. This report contains detailed profiles of twelve of the world's leading platform companies and derives insights from those profiles about what platforms actually do, how they do it, and why they succeed financially. For example, the report finds that although platforms tend to have a number of economic characteristics in common, they also vary so greatly that they cannot be compartmentalised into just a few categories, let alone a single sector. Moreover, they do not all succeed for the same reasons. In addition, although the major Chinese platforms still have a low profile within the OECD, they are in the process of expanding globally and deserve more attention.