This paper discusses the growth of mathematical understanding of two students, Graham and Don, as they use a computer graphing program to explore the properties of quadratic equations. Through analysing extracts of video data using the Pirie Kieren theoi T, we discuss the ways in which the mathematical understanding of the students grows and how their interactions occasion, facilitate, and restrict this. We consider four 'clips' of their mathematical working, highlighting different aspects of their developing understanding, and use of the graphing software. Although we are talking about a computer based graphing package, our conclusions are equally relevant to the use of graphing calculators.Penglase and Arnold (1996) provide a full and critical review of recent research into the use of the graphical calculator in mathematics teaching and learning. In considering the particular use of technology for exploring functions and graphs they note that: Some evidence suggests that use of the graphics calculator" aids the development of a more global understanding of the featm'es of functions, encore'aging conceptual images of functions, and understanding of the relationship between functions and graphs. Other" evidence appears to demonstrate that its use can result in incomplete understandings of function concepts. (p. 66) Interestingly, Penglase and Arnold (1996) note that many of these contradictory findings derive from the approaches which have been taken in looking at whether a graphing program leads to "a better understanding of the relationship between an algebraic function and its graph" (p. 67). They suggest that claims about the effectiveness of the software are often based on assessment procedures which equate understanding and learning with performance on traditional tests. Further, they suggest that much research is not actually concerned with the use of the tool in the process of teaching and learning, but instead a form of "program evaluation" (p. 58). In this paper we do not wish to equate mathematical understanding with test performance, and hence are not trying to evaluate the use of the computer in such terms. We are, however, seeking to offer an insight into the learning processes through which the computer might enable learners to work towards a better understanding of the relationship between a function and its graph.Hang and Thomas (1997), in investigating the effect of the use of computer based instruction, note that students who have used technology (in their case to work on calculus) developed a greater degree of "conceptual awareness" in comparison to those who were taught in a standard lecture form. Hang and Thomas (1997) suggest that without the computer the students had gaps in their conceptual understanding, and saw calculus as a "series of procedures and algorithms" with no "versatility of thought" (p. 87). They argue that computer based curriculum materials can give an "improved cognitive base" leading to a more powerful and flexible understanding of the mathematical concept. In a simila...