The question of additionality is easy to understand and difficult to apply. This paper presents new methodological approaches for assessing the additionality of climate investments, focusing on the Climate Public Private Partnership (CP3), a £130m program funded by the UK’s International Climate Fund. One approach is qualitative and uses a multi-criteria assessment (MCA) framework applied through case studies. The other is quantitative and explores additionality through a composite index that assesses the investment environment in countries where investments took place. These approaches have different aims and are complementary, allowing evaluators the ability to assess the additionality of specific investments undertake portfolio-level assessments.