This chapter covers the establishment, contents and implementation of international environmental liability of the contractor. It argues that the polluter-pays principle applies, the contractor shall assume primary liability for environmental damage caused by DSM activities. However, unlike the existing civil liability treaties which endorse a standard of ‘strict liability’ of the operator, liability in DSM requires an element of ‘internationally wrongful act’ on the part of the contractor. As to the contents of liability, it argues that ecological restoration shall be the primary objective of liability. However, owing to the big unknowns of the deep sea, the practicability of restoration is very uncertain. Other forms of remediation, complementary restoration and compensation, are available. As to the implementation of liability, this chapter argues that the ‘traditional civil liability approach’ of a transnational nature is not suitable to the DSM context, and the parallel existence of implementation methods at both international and national levels weakens the international mechanism and places unnecessary burden on the sponsoring State. Incidentally, it argues that the Enterprise can assume liability independently and a State contractor cannot claim State immunity.