Open Source Software (OSS), an example of an IS innovation, provides an alternative to proprietary software for organizations. Despite its free availability, OSS has not been universally adopted. While IS innovation has been extensively studied, there is a dearth of research literature on the adoption of OSS. Using a multi-site case study research method and a well known framework on the adoption of IS innovations, we explore this novel phenomenon and uncover critical factors that influence the adoption of OSS in organizations.