Balancing the competing goals of collaboration and security is a difficult, multidimensional problem. Collaborative systems often focus on building useful connections among people, tools, and information while security seeks to ensure the availability, confidentiality, and integrity of these same elements. In this article, we focus on one important dimension of this problem---access control. The article examines existing access control models as applied to collaboration, highlighting not only the benefits, but also the weaknesses of these models.