The constructs of "common ground" and "grounding" are frequently invoked in the CSCW literature as a mechanism by which participants engaged in joint activity coordinate their respective understandings of matters at hand. These constructs arise from a model of conversation developed by Herbert Clark and sometimes referred to as "contribution theory." We describe here the basic features of this theory and attempt to apply it in analyzing a fragment of enacted interaction. The interaction was recorded during an abdominal surgery performed with the aid of an endoscopic camera. We encountered difficulties, however, in applying contribution theory as an analytic framework within this concrete setting. We found further that the notion of common ground represents a confusing metaphor rather than a useful explanatory mechanism. We conclude with a suggestion that researchers in the future seek ways of constructing descriptions of joint activity that do not rely on ...