CSCW seems to have a persistent problem of understanding the nature of "cooperative work". This paper argues that this problem is the direct result of not looking at the dynamic aspects of work — i.e. that cooperative work is not one thing, but different things at different times in different places. Based on Activity Theory the paper gives a conceptual framework for understanding the dynamics of collaborative work activities, and argues that the design of computer support should view cooperative breakdowns not as a problem but as an important resource in design. These arguments are based on empirical studies of healthcare work and the design of a computer support for planning and scheduling operations and other activities within a hospital.