This was an evidence submission to the UK Parliament's International Development Committee. I argue that while BII (British International Investment) has been a leader amongst development finance institutions (DFIs) in important respects, and has a strong staff and management focused on improving development outcomes, it still suffers from constraints that limit the impact of DFIs as a whole in supporting sustainable development, especially in the poorest countries. Like other DFIs, BII is unable to mobilize significant capital by crowding in private finance to its investments, its efforts to create new markets for private finance have had limited success and its use of subsidized capital has been inefficient.