In this paper I highlight a few important aspects of the roles that external representations can play in the meaning-making activities of two or more participants, and how the particular representation used may influence these activities. I will draw upon my own research for examples. One line of inquiry showed how the properties of representations used to support evidence-based inquiry can affect both discourse activities and student work. A second line of inquiry showed that the distinction between communication tools and other representations is blurred online, with the actual discourse between participants being accomplished by actions in all of the mutable representations. This research is unified by the concept of representational guidance: the constraints, visual properties, and conventions of use associated with representations can be used to guide learning activities.