Sustainability impact assessments (SIAs) act as bridges between trade and investment agreements and social, environmental and human considerations. They are relevant as inputs into the treaty negotiation process and as interpretive aids in investment treaty arbitration. As inputs, SIAs attempt to measure the impact of environmental, social, economic and human rights aspects of trade and investment agreements prior to and during a treaty’s negotiation. SIAs have been performed on all major negotiations in the EU since 1999, and will continue to be performed under its investment competence. Case studies in this article demonstrate how SIAs may include climate change. Additionally, legal recommendations are offered which, if adopted by the Directorate General for Trade, should increase the effectiveness of SIAs. As interpretive aids, SIAs may be key references in treaty interpretation arguments in light of increased environment and development related investment disputes and focus on sustainable development policy space.