Why do biotech firms cluster? New and established firms in biotech clusters are said to capitalize on knowledge spillovers, labor-market pooling, and other externalities. Some have even argued that such spillovers are so strong that the cluster itself, rather than the individual, is the locus of entrepreneurship. Such arguments, however, do not resolve the mechanism by which clusters might contribute to the establishment of new firms. This paper proposes a conceptual framework for analyzing the locational choices of entrepreneurial firms in the life-sciences industry. Building on both the cluster literature and the literature on entrepreneurship, we develop hypotheses about how cluster characteristics, the entrepreneur's personal characteristics, and characteristics of the business environment affect the entrepreneur's decision to establish a new firm. We argue that a key factor in the location decision is the relative mobility of the appropriate resources. Our main hypothesis is that specialized labor is less mobile than capital and other resources and that it is the base from which entrepreneurs are ultimately created. If so, new firms will emerge in areas characterized by an existing concentration of specialized labor. This labor pool may be spawned by universities and incumbent small and larger biotech (or other high-technology) companies. An alternative explanation is that entrepreneurs establish new ventures outside the cluster, then move them to the cluster to take advantage of local knowledge and other resources. Or a potential entrepreneur could conceive a business plan, then relocate to an existing cluster before founding a firm. We explain how survey data can be used to sort through these explanations.