Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, are provided for handling user-created advertisements. In one aspect, a method includes receiving data from a first end user to create a first advertisement; receiving targeting criteria associated with the first advertisement, the targeting criteria indicating a second end user to whom the first advertisement is to be shown: receiving, from a client device, a first indication of the second end user browsing a webpage; and providing the first advertisement to be displayed to the second user on the webpage upon receiving the first indication.