In a two-day workshop, 12 teachers who had not previously used graphics calculators not only learnt how to use a calculator but also completed calculator tasks designed to explore their misconceptions and then learnt about students’ difficulties. Observation of lessons taught subsequently by 6 of the teachers, and interviews with 15 of their students, were used to investigate the effectiveness of the workshop in promoting appropriate use of technology in their mathematics classes. It was found that the teachers consistently drew their students’ attention to limitations of the technology. Apparently as a result, their students showed considerably fewer misconceptions than students in a previous study who had been taught by teachers who had not undertaken such professional development. The few remaining student misconceptions could be traced to lingering uncertainties on the part of their teachers. Implications of the findings for mathematics teacher education are discussed.