If the world is going to make significant progress towards achieving the Sustainable Development Goals (SDGs) by 2030, development actors will need to think and work in new ways, including in evaluation. We believe that complex systems-informed approaches can make a major contribution. With reference to SDG14 (life below water), this briefing offers two examples: (i) exploring interactions between SDG targets, and (ii) shifting attention from projects and programmes to systems. Such approaches can help all development actors — including monitoring, evaluation and learning specialists — to create boundary-spanning development and evaluation plans, identify leverage points, priorities and trade-offs, and reveal new ways to accelerate progress.