There is widespread agreement that further research is needed in order to identify afterschool program characteristics useful for understanding why some programs are more successful than others. The bulk of recommendations put forth by researchers, practitioners and policy makers focus on observable characteristics of the afterschool setting as a whole. While these characteristics can be recorded on checklists for later aggregation into a quantifiable evaluation of the system, it is important to remember that they are the products of interactional processes. In the present analysis I focus on the dynamic human interactions that comprise these system-level evaluations. Drawing on video documentation of adult-child computer mediated activities in a UC-Links afterschool program, I illustrate how UC-Links design principles -- which focus on the creation of "cultures of collaborative learning" -- promote the learning and development of participating youth. In particular, I show how implementation of these principles support one of the key tasks in achieving quality teaching-learning after school: the successful negotiation of a common ground of engagement between interlocutors in an instructional interaction.