Tabletop groupware systems have natural advantages for collaboration, but they present a challenge for application designers because shared work and interaction progress in different ways than in desktop systems. As a result, tabletop systems still have problems with usability. We have developed a usability evaluation technique, T-CUA, that focuses attention on teamwork issues and that can help designers determine whether prototypes provide adequate support for the basic actions and interactions that are fundamental to table-based collaboration. We compared T-CUA with expert review in a user study where 12 evaluators assessed an early tabletop prototype using one of the two evaluation methods. The group using T-CUA found more teamwork problems and found problems in more areas than those using expert review; in addition, participants found T-CUA to be effective and easy to use. The success of T-CUA shows the benefits of using a set of activity primitives as the basis for discount usabil...