Abstract. For unknown reasons, research into ubiquitous computing seems to pay no attention to collaboration among users in such an environment. This paper presents the design philosophy of activity-based computing (abc) and a technical implementation of it in a ubiquitous computing infrastructure, the ABC framework, which takes collaboration in ubiquitous environments as its starting point. The idea of activity-based computing and the aim of the framework is to: (i) support the human activity by managing its collection of work tasks on a computer, (ii) to support mobility by porting activities across heterogeneous computing environments, (iii) to support asynchronous collaboration by allowing several people to participate in an activity, and (iv) to support synchronous, real-time collaboration by enabling 'desktop conferring' by sharing the activity across several clients. During a period of two years, our framework have been co-designed and evaluated in cooperation with a range of clinicians in a hospital.