Expertise is regarded as the most important asset for groups working on knowledge-intensive tasks. This dissertation advances a multi-dimensional conception of group expertise that includes depth, breadth and variety as three distinct dimensions, and develops a model of the joint effect of the three dimensions on knowledge sharing and group performance. Two empirical studies test different components of the model. Using data on 174 groups from a Fortune 500 telecommunications company, one study supports the hypothesis that depth of group expertise will have a less positive effect on group performance as breadth of group expertise increases, particularly when groups engage in a low level of external task process. Using data on knowledge sharing networks among 44 employees of a supply chain service organization, the second study probes into dyadic interactions underlying group process, and tests how individual variety of experience, functional department experience (proxy for depth of expertise at the individual level), and dyadic functional difference (proxy for breadth of expertise at the dyadic level) affect dyadic knowledge sharing. Results show that individual variety of experience improves ease of knowledge sharing, and that the negative effect of functional department experience on ease of knowledge sharing is mitigated by individual variety of experience. Theoretical and practical implications of the findings are considered, and directions for future studies are discussed.