Online communities are fast becoming a major aspect of the daily lives of millions of people around the world. Determining what makes an online community successful is therefore crucial for developers and designers. Why do some succeed and some fail? In order to study these questions, an online academic community was created for students in an interdisciplinary doctoral program at a major university in the United States. This study reveals that a narrow purpose appears to be the strongest indicator of success for an online community. The purpose brings them in to the community.