Platforms that connect users to one another have flourished online in domains as diverse as transportation, employment, dating, and housing. When users interact on these platforms, their behavior may be influenced by preexisting biases, including tendencies to discriminate along the lines of race, gender, and other protected characteristics. In aggregate, such user behavior may result in systematic inequities in the treatment of different groups. While there is uncertainty about whether platforms bear legal liability for the discriminatory conduct of their users, platforms necessarily exercise a great deal of control over how users' encounters are structured—including who is matched with whom for various forms of exchange, what information users have about one another during their interactions, and how indicators of reliability and reputation are made salient, among many other features. Platforms cannot divest themselves of this power; even choices made without explicit regard for discrimination can affect how vulnerable users are to bias. This Article analyzes ten categories of design and policy choices through which platforms may make themselves more or less conducive to discrimination by users. In so doing, it offers a comprehensive account of the complex ways platforms' design and policy choices might perpetuate, exacerbate, or alleviate discrimination in the contemporary economy.