This report details the range of existing and emerging risks that BP and Shell face from their expansion of production in the Canadian tar sands. We believe these risks are significant for BP and Shell shareholders and that investors should question the companies more deeply on their tar sands strategies and call for greater transparency regarding the assessment of the mid to long term viability of these projects. Investors should call for full disclosure of the risks involved in the tar sands strategy in a carbon constrained world and the development of new tar sands projects should be halted.