Tools for shared critical reasoning are the focus for much innovation in computer supported collaborative learning. Argument diagramming tools allow learners to create shared graphical depictions of their shared reasoning; celebrated exemplars are recognized by their distinctive node and link formats. Many evaluations of these tools have been published, describing their effects on collaboration and learning and offering explanations of the features and mechanisms that engender better argumentation. It would be useful then, to review these evaluations to try to discern common themes and findings, and to understand the different approaches taken. This chapter reviews significant evaluations of collaborative argumentation diagramming tools, it summarises important findings about the features of the tools and their effects on collaboration and learning. The approaches taken in the evaluation of the tools are considered in relation to their assumptions and aims, methods and outcomes.