This paper examines the extent to which mathematical knowledge, and its related pedagogy, is inextricably linked to the tools--physical, virtual, cultural--in which it is expressed. Our goal is to focus on a few exemplars of computational tools, and to describe with some illustrative examples, how mathematical meanings are shaped by their use. We begin with an appraisal of the role of digital technologies, and our rationale for focusing on them. We present four categories of digital tool use with their differing potential to shape mathematical cognition. The four categories are: (1) dynamic and graphical tools, (2) tools that outsource processing power, (3) new representational infrastructures, and (4) the implications of high-bandwidth connectivity on the nature of mathematics activity. In conclusion, we draw out the implications of this analysis for mathematical epistemology and the mathematical meanings students develop. We also underline the central importance of design, both of the tools themselves and the activities in which they are embedded.