A new research agenda within political science and economics has been devoted to elucidating the development of medieval representative institutions. However, if we wish to get a clearer grasp of the birth of representative institutions, much more detailed data are needed. This paper introduces and interrogates a new data set on assemblies in one of the first instances where representative institutions arose: the Crown of Aragon. An analysis of these new data, which cover the period 1100–1327, supports several important conclusions. First, the development of representative institutions took place in a couple of brief "bursts" of activity. Second, factors pertaining to the establishment of public order (land peace and the regulation of succession) feature as more important than "geopolitical" factors (war and taxation) in the early part of the period, but the latter factors become more important after assemblies turn representative; indeed, they turn out to be crucial for understanding how assemblies eventually came to constrain rulers. A comparison with the only other early case of representative institutions on which there are good data, thirteenth- and early-fourteenth-century England, supports a third finding: There is a striking, temporal parallelism in the development of representative institutions across the two realms.