An ad-selection analysis subsystem (“analysis subsystem”) analyzes advertisement selection data to identify relevant queries for advertisements. Advertisement selection data for each advertisement in a set of advertisements are represented as a vector of terms corresponding to search queries for which the corresponding advertisement was provided with search results and, in turn, selected by a user. A clustering algorithm is applied to the advertisement selection data for the set of advertisements to identify clusters of search queries and corresponding clusters of advertisements. Identified clusters can be used, for example, to facilitate query expansion, advertisement selection, and keyword generation.