Most research on the emergence of high-tech clusters samples on successful cases, and works backwards to trace a narrative, often highlighting the role of specific individuals or groups. Our approach begins with the formation of a new field--biotechnology in the late 1970s and early 1980s, and follows the field to the present. We emphasize the sequence of network formation, and the importance of organizational diversity and catalytic organizations that provide relational and normative glue. We examine eleven regions in the US that were rich in resources--ideas, money, and skills

--that could have lead to the formation of life science clusters. Three of the communities formed robust groupings, but most did not. Although local details are always relevant, our argument transcends the nuances of history in each community to specify the processes and mechanisms that foster catalytic growth. The necessary conditions are a diversity of for-profit, nonprofit, and public organizations, a local anchor tenant, and a dense web of local relationships. These features make possible cross-network transposition, whereby experience, status, and legitimacy in one domain are converted into ‘fresh’action in another. The argument does not hinge on specific types of organizations or ingredients; indeed, it is general enough to accommodate multiple pathways.