While the issue of teachers' perspectives on the barriers to technology use has received considerable attention, teacher concerns have not been studied in a systematic and holistic way. The present paper examines teacher concerns regarding a proposed technology-based innovation using Activity Theory as a theoretical framework. Fifty-one teachers participated in an inservice blended learning course in which a real-life Computer Supported Collaborative Learning example was presented and subsequently discussed in the online forum. The teachers were asked under which conditions they thought Computer Supported Collaborative Learning could be incorporated into their daily practices. Teacher concerns were identified through their online posts. Time and curriculum constraints were reported by teachers as the main obstacles to the proposed technological innovation. The examination of these obstacles using Activity Theory helped to identify three major contradictions in teachers' activity system: (a) within the object of activity, (b) between the mediational means and the object of activity and (c) between the current and the proposed object. These contradictions are discussed from the perspective of Activity Theory and their implications for the design and implementation of technology are drawn. (Contains 1 table and 3 figures.)