A feature of certain investment treaty claims is that they can appear to bring into conflict obligations owed by States under international investment agreements (‘IIAs’) and obligations owed by States under other treaties or under customary international law. This chapter examines arbitral tribunals’ consideration of the core standards of protection in IIAs in cases which give rise to environmental issues. The analysis reveals that although environmental issues have loomed large in the factual background to these disputes, such issues have generally played only a supporting role (if any) in the final decision. It is difficult to predict whether obligations under international environmental law may ever be determinative in the outcome of investment disputes. But the possibility of such a result cannot be discounted, as public international law has tools which may be used to enhance the role of environmental issues in the resolution of such claims.