The chapter looks at the clash between international investment agreements (IIAs) and protection of the environment. International investment law has revealed all its potentialities – and its negative impacts – through many investment arbitrations related to host State’s environmental regulations. The chapter details the terms of the tension between investment and environment and drafts some propositions to reconcile the two. Therefore, it analyses the main types of environmental provisions in IIAs and proposes to go further in the greenization of IIAs, with propositions turned on the investor’s responsibility. Another section examines the articulation between the substantive protection standards in IIAs and the right to regulate of the host State in the field of environment. Finally, it is examined if investor-State dispute settlement mechanisms are suitable to settle environmental-related disputes.