This article examines teachers' 'practical theories' concerning the contribution of information and communications technology (ICT) to teaching and learning, as they are elaborated and refined in action. The study arose from a collaborative programme of 10 small-scale projects through which participating teacher-researchers aimed to develop a range of pedagogic strategies involving the use of computer-based ICTs within their subject areas (Classics, English, Geography, History, Science and Design Technology). Within- and cross-case analyses drew on multiple sources of data and characterised teachers' initial statements of practical theory in terms of five key themes: broadening classroom resources and reference; enhancing working processes and products; fostering more independent pupil activity; mediating subject thinking and learning; and improving pupil motivation towards lessons. Three cases are presented to illustrate how teachers developed their ideas in action, and issues that prompted teachers across cases to adapt their approaches are discussed.