This chapter analyses the establishment, content and implementation of international environmental liability of the sponsoring State. First, it argues for a differentiation between State liability ex delicto and State responsibility and suggests to employ the former notion. Second, it points out the non-practicability of an application of the full reparation principle (restituto in integrum) for environmental damage and that liability of sponsoring State should be taken together with that of the contractor. Third, it argues that invocation of liability of the sponsoring State for environmental damage by the ISA is preferable to that by States (parties). As to liability of the ISA, it focuses on questions of whether and, if so, in what situations member States are held liable for the ISA. Based on analysis of the 2011 ILC Articles on Responsibility of International Organization, 1995 IDI Resolution and 2004 ILA Resolution, it finds out that there is a ‘no liability of member states’ rule which however allows for exceptions. It also discusses invocation of liability of the ISA.