Research into the sources of productivity growth has taken many disparate routes, from aggregate theoretical modeling to firm-level empirical explorations. This article provides examples from the literature of evidence against the representative firm assumption. Once heterogeneity of firms and market selection is taken into account, theoretical effects of policy on aggregate productivity follow through many paths. The article gives examples of recent empirical work that uses micro-level information to assess the effects of policy on macro-level productivity. © The Author 2010. Published by Oxford University Press on behalf of Associazione ICC. All rights reserved.