This thesis examines the impact that collegia had on the economy of the western Roman Empire. There has been a tendency amongst scholars to dismiss the economic aspects of collegia and to think of them instead as primarily social or religious organisations. This thesis demonstrates, however, that some of the earliest work on collegia was affected by flawed methodology and that this has in turn affected a good deal of the subsequent scholarship, undermining economic analyses to a certain extent. As a secondary research goal, this thesis is therefore aimed at re-examining the body of scholarship that has gone before it in light of modern research methods and in hopes of drawing a line under many of the debates that continue to be given too much prominence in the literature. In examining the economic impact of the collegia, the thesis draws on modern economic theory and especially on a Neo-Institutional Economic theoretical framework to analyse the epigraphic and, latterly, the papyrological evidence of associations in the Roman Empire. The conclusions demonstrate that many of the collegia in the west did have a clear impact on their local economies, despite this not always being immediately discernible in the extant epigraphic evidence. By also examining other types of evidence, namely papyri and the archaeological evidence from Ostia, however, we are able to significantly enhance our understanding of the collegia.