We examine the evolution of biotechnology clusters in germany between 1996 and 2003, paying particular attention to their composition in terms of venture capital, basic science institutions, and biotechnology firms. drawing upon the significance of co-location of "money and ideas", the literature stressing the importance of a cluster's openness and external linkages, and the path dependency debate, we analyze how certain cluster characteristics correspond with each cluster's overall performance. after defining different cluster types, we explore and compare the clusters' internal and external interconnectivity, and investigate the extent to which the clusters' structural composition has changed over time. our results indicate that the structure, i.e., the type of cluster, and the cluster's openness towards receiving external knowledge provide merely unsystematic indications of a cluster's overall success. the cluster's ability to modify its composition to a more balanced ratio of science and capital over time, in contrast, turns out to be a key explanatory factor. Jel-classification: l22, o18, o32.