Substantial effort in the development of any large system is invested in testing. Studies of testing tend to be either technical or concerned with the cognitive ability of testers. Our experience is that testing is not technical but socio-technical, involving a great deal of human and organisational effort, and that testing is not simply the kind of decontextualised `puzzle solving' many cognitive approaches imply. We believe that cooperative work is foundational to getting testing done. In this position paper, we use data from four ethnographic studies to discuss just what that cooperative work is. Categories and Subject Descriptors D.2.5 [Testing and Debugging] General Terms Human Factors Keywords Ethnography, Testing, Cooperative Work.