Secondary-school systems throughout the world are preoccupied with technology integration in subject teaching and learning. Advocacy of the educational use of new technologies often seems to suggest that their value is evident, their adoption urgent, their implementation unproblematic, and their impact transformative. However, the recent TIMSS-2003 studies show that technology integration is extremely rare in school Mathematics and Science, even when national curricula make reference to the use of computers, equipment is available for classroom use, and teachers have received recent training in technology integration in their subject. Drawing on studies of how mathematics and science teachers in England see technology use as contributing to their practice, this paper shows how established subject cultures--notably their pedagogical discourses and practices--shape the "practical theories" that teachers bring to ICT use--illustrating this through a specific example of teachers' use of dynamic geometry software. These English studies of informally developed technology use by teachers are then compared with ideas emerging from French studies of project-based development of technology use. The paper concludes by arguing that the challenge of technology integration within an institutionalised community calls for a dialogic process of development in which current practice is sympathetically analysed and imaginatively reviewed in the light of speculative possibilities to produce viable new practical theories. (Contains 2 tables and 3 figures.)