The OECD DAC Blended Finance Principles for Unlocking Commercial Finance for the SDGs give a clear definition and provide a five-point checklist to ensure blended finance meets accepted quality standards and achieves impact, based on a development rationale promoted by Development Assistance Committee (DAC) members. These Principles represent a critical first step towards ensuring that donors engage in the right way, by guaranteeing that the policy framework is fit for purpose. Whether a donor is looking to begin a blending programme or already has one, we recommend that the OECD DAC Principles be used to test assumptions about how blending is currently being undertaken on critically important aspects such as the engagement of local capital markets, the use of concessionality as a precondition to blending, or the provision of monitoring and evaluation mechanisms.