Methods, and systems, including computer programs encoded on computer-readable storage mediums, including a method for determining a relevance of search keywords to an advertisement. The method includes accessing ad keyword data specifying an ad keyword associated with an advertisement; identifying a keyword cluster specifying terms related to a topic of the advertisement; determining whether the ad keyword occurs in the at least one keyword cluster; identifying one or more search keywords to which the advertisement is responsive; determining whether the one or more search keywords occur in the at least one keyword cluster; and determining, by one or more data processors, a relevance of the one or more search keywords to the advertisement based on the determination of whether the ad keyword occurs in the at least one keyword cluster and the determination of whether the one or more search keywords occur in the at least one keyword cluster.