A method of classifying content items utilizes user-generated playlists. A content item is included in playlists from a plurality of distinct users. A server system receives, from respective users of the plurality of distinct users, respective playlist titles for the playlists. In accordance with a determination that the respective playlist titles from a threshold number of the plurality of distinct users specify a first attribute, the server system assigns the first attribute to the content item. The server system receives a request, from a first user via an application that is executing on a client device that is distinct from the server system, for content having the first attribute. In response to receiving the request, the server system selects the content item according to the first attribute and delivers the selected content item for playback to the first user.