["The RAND Journal of Economics, EarlyView. ", "\nABSTRACT\nThis article provides a full characterization of the competitive effects of horizontal mergers in the Cournot model with heterogeneous firms and constant marginal costs. We show that price effects depend only on the smaller merging firm's market share and the number of firms but are independent of the distribution of market shares among other firms. Standard concentration measures, instead, are often misleading. We also provide simple—yet general—closed‐form solutions for merger effects based on pre‐merger parameters. Moreover, we extend the model to study the combination of output and input market power and relate our results to the Merger Guidelines."]