In Western Europe, cities grew fast between 1000 and 1300. This article looks at how the migrants moving to these emerging cities melded into communities and defended their collective rights in violent and hostile environments. It discusses a number of trust networks that were developed to meet the successive collective needs of particular social layers in rapidly changing urban societies. When a trust network became well-established and too large to allow face-to-face relations among all its members, institutions were created that (...) operated under different rules. (shrink)