The "T-MEDIA" project analysed and documented how teachers exploit the use of projection technologies--data projectors and interactive whiteboards (IWBs)--to support learning in secondary-school subject lessons. The research involved collaboration between university researchers and four pairs of UK teachers (of English, mathematics, science and history) in an intensive process of systematically analysing video recordings of classroom activity and other data in depth. Our goals were to assist teachers in articulating the pedagogical rationale underlying their practice, and uniquely, to engage them in theory building about strategic technology use. This paper reports on a follow-up study carried out one year after the collaborative analyses in order to assess the subsequent impacts of the process of critical reflection. The eight teachers were questioned using a semi-structured interview technique that allowed us to elicit structured and personalised accounts of impact on pedagogical thinking and practice and the supporting or constraining factors. The findings suggest that for at least some, the sociocultural theory introduced and reformulated during the analyses provided a powerful analytical lens upon emerging practices, including those not incorporating technology. All of the participants reported lasting effects upon their own thinking and, except where external constraints operated, on teaching practices. The approaches developed during T-MEDIA had additionally been disseminated to and adapted by other subject colleagues. The study illustrates how collaborative analysis of lesson videos can be used to engage teachers in deep reflection, critique and debate. This approach supports the development of an analytical scrutiny of classroom teaching and offers a significant professional development opportunity. In particular, under conditions of sensitive support, teachers will readily accommodate theoretical constructs into specific areas of professional thinking and practice. (Contains 2 tables and 12 notes.)