The role of an educational technologist is difficult to define. This paper reflects on the experience of working on a range of technology enhanced learning in science projects to review a number of working principles which have proved effective in the practice of educational technology. It discusses how these principles relate to the theories in use in educational technology. Three case studies are considered illustrating methods of participatory design, design based research and socio-cultural approaches. All the case studies are taken from the application of educational technology principles to the design of examples of science instruction. The paper offers an interpretation of the contemporary practice of educational technology. (Contains 2 figures.)