Silicon Valley is the most salient example of high-tech industrial clusters. Public policymakersthroughouttheworldwouldliketolearnthesecretsofSiliconValley in order to build their own high-tech economies. The existing literature on ind- trial clusters, which traces back to Marshall (1920), focuses on the way in which ?rms bene't from locating in a cluster; it suggests that once a cluster comes into existence, it tends to reinforce itself by attracting more ?rms. However, a more important question is how to reach this critical mass in the ?rst place. In contrast to the literature, evidence...