Lately we have seen a growing interest from both public and private organisations to adopt Open Source Software (OSS), not only for a few, specific applications but also on a more general level throughout the organisation. As a consequence, the organisations’ decisions on adoption of OSS are becoming increasingly more important and complex. We present three perspectives organisations can employ in their decisions: seeing OSS acquisition as a business case, as COTS acquisition, and as architectural change within a governance framework. We present case studies of decisions on OSS adoption, and categorise the decision criteria we have found. Our results indicate that for large-scale adoption of OSS, focus will be on architectural considerations: enterprise-wide architectures will at first be a barrier, but in the long term OSS’s support of open standards can be a major enabler for OSS adoption. In contrast, in smaller organisations and in small-scale adoption of OSS, the cheap price of OSS is a major enabler, as it provides a good opportunity for experiments and short-term economic benefits. For small organisations these experiments can lead to development of a common IT-architecture, and in larger organisations OSS can be adopted in niche-areas, without significantly violating an existing IT-architecture.