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.In his last manuscript, Charles Tilly developed the concept of trust networks as organizations mediating in the long-term transformation from urban to state domination in world history. Trust networks can be defined as specific patterns of human relationships in which shared interests, values and norms facilitate communication and collaboration and thus reduce transaction costs. Trust networks are based on commitment, coordination, and recognition which often, but not always, are mutual. Membership of a trust network entails a "relation in which at least one party places a valued enterprise at risk in the hands of the other.… Prudent members of trust networks have generally tried to insulate them from predation or incorporation by cities and states (Tilly 2010)." In this article, I show how trust networks emerged and became established in the cities of Europe between about 1000 and 1600 CE. In my view, community building in the first stage of urban growth rested essentially on personal ties, formalized by oaths of loyalty. These were at the origin of the sense of belonging expressed in the first communes, the groupings that fought for their independence vis-à-vis the established feudal and ecclesiastical powers. As the European cities grew, these networks needed to organize themselves as public institutions, which allegedly changed their character Theor Soc (2010) 39:315-326