When technologies are used in mathematics education it is important to assess how activities will support the development of mathematical understanding and the technical expertise that students will need. The two issues are discussed in this paper. Literature is reviewed and findings from a recent study in a high-school class are reported. The students were studying descriptive statistics. The teacher introduced regression using dynamic graphs on Java applets. The approach seemed conducive to understanding. Graphics calculators, which all students owned, were used infrequently for investigating properties and were never used to introduce new constructs--one reason was that the static graphs on the calculators did not support inference of basic statistical relationships well. However, substantial time was given to developing competence with operating the calculators, which was necessary for success in public examinations. The outcomes, overall, indicated that ongoing assessment of decisions about technology-use should occur at system level as well as in the classroom.