Methods, systems, and apparatus, including computer programs encoded on a computer program product, for selecting advertisements. In one aspect, a method includes receiving publisher selections of advertisements; associating the selected advertisements with an advertisement environment in a document; generating an advertisement request code for inclusion in the document; evaluating performance metrics for the selected advertisements; and in response to determining that the selected advertisements do not meet the performance threshold, optimizing the selection of selected advertisements based on the performance metrics; substituting a selected advertisement with a candidate advertisement and causing a client device to render the candidate advertisement in the advertisement environment in the document.