²See for instance Brülhart, Sbergami (2009) as a recent analysis of the effects of agglomeration at the national level. Duranton, Puga (2004) and Rosenthal, Strange (2004) provide a good theoretical survey on micro-foundations of agglomeration economies, and an extensive review of the empirical evidence. Spence et al. (2009) provide a comprehensive review linking the literature on agglomeration economies with the literature on urbanisation and growth.