Collaboration is increasing prevalent among knowledge workers and gives workers access to the expertise, experience, ideas, and perspectives of their colleagues. Although scholars have noted that collaboration is one way through which workers can develop their human capital and improve their work performance, little consideration has been given to the interdependence that this creates between them and their careers – and, more specifically, what happens when collaborators change jobs and move to another organization. This dissertation addresses this gap by considering the consequences of colleague exit for a knowledge worker’s performance, career mobility and the formation of new collaborative relationships. First, I present a theoretical perspective on the impact of colleague exit that focuses on a focal worker’s access to the knowledge of others and its implications for the worker’s performance. This draws attention to the nature of the exiting colleague’s relationship with the focal worker, in addition to the strength of that relationship and the value of the exiting colleague’s knowledge resources. I then explore the consequences of colleague exit for a focal worker’s career mobility and formation of new relationships, using a dataset that combines internal HR data, publications data, and other hand-collected data on employees engaged in scientific research within a single organization. I argue that the exit of colleagues who are collaborators will reduce a worker’s existing opportunities to develop their human capital and improve their performance, encouraging worker turnover. Yet, collaborator exit may also generate opportunities for new collaborative relationships, mitigating the negative effects of collaborator loss and even increasing the likelihood of promotion among workers who stay. My findings support these arguments and show how the impact of colleague exit on a worker’s mobility varies depending upon the worker’s relationship with exiting colleagues, that is whether or not they are collaborators, and the characteristics of those leaving. I also find evidence that remaining workers are more likely to forge new collaborative relationships in the wake of collaborator exit, and go on to consider the role played by collaborators-in-common who serve as indirect ties between them.