Project-based organizations allow employees in ostensibly similar roles to acquire very different experiences by working on different kinds of projects. We study how people build careers in these contexts, examining when employees choose to diversify their experience-both in terms of project content and collaborators-and how the resulting diversification affects their career advancement. Using longitudinal project data from a services organization, our results suggest that employees initially explore different kinds of work by moving across project types, but then go on to find their fit in a particular area. This process is quicker for high-performing employees and for those with longer tenure and more diverse collaborator networks. We also find that promotion rates and compensation are lower for employees who worked on a broader portfolio of content types and collaborators in the most recent year, but higher for employees who had worked on broader project portfolios in prior years.