This chapter finds that the ISA defines ‘marine environment’ broadly and imposes a threshold requirement – ‘serious harm’ – to the definition of ‘marine environmental damage’ in DSM. It argues that the determination of whether the threshold has been crossed should be based on both factual assessment and value judgment. The measure of marine environmental damage in DSM is a much more complex issue. This chapter draws on experience in other contexts, including the International Oil Pollution Compensation Funds, the ‘F4’ claims before the United Nations Compensation Commission, the US and EU laws and practice and discussions in the context of the Convention on Biological Diversity. Upon examination, it proposes to rely primarily on a cost-based method, while valuation of pure environmental damage or interim loss secondary. It suggests a two-step pattern for measuring environmental damage in DSM.