It can be expected that once increasing supply risks are appropriately communicated and major supply vulnerabilities are manifest, actors in importing countries will enact public and private measures to respond. An early attempt to use Stirling’s SWI to assess security of supply by an international organisation was made by the IEA in Towards a Sustainable Energy Future (IEA, 2001). The SWI remains a useful tool, but as has been pointed out above it must be complemented by qualitative information about different supply options. Well-diversified, liquid markets may provide a hedge against the exercise of market power. Competition authorities tend to focus on the related but opposite characterisation of markets: the extent of market concentration. The most common indicator used to measure market concentration is the HHI developed above.