Interest rates have been a contentious issue in microfinance for many years. While higher interest rates for microloans are often justified by the underlying costs of making small loans in rural areas, this is not always the case. Furthemore, this issue is especially relevant to the impact investors such as impact investment funds and development finance institutions that fund microfinance institutions (MFI) to expand lending to smallholder farmers and other micro-borrowers in the agrifood system. Questions about the adequacy of interest rates charged by MFIs targeting farmers often ariseduring the due diligence (DD) process and can provoke animated discussions during investment committee meetings. The frequency and intensity of these debates at multiple investment committee meetings of impact investment funds co-financed by the European Commission actually prompted the idea to produce this technical guide.This toolkit provides guidance on how to analyse interest rates of MFIs from a responsible lending perspective and how to strengthen responsible lending practices more broadly. It is mainly targeted at impact investors and other financiers with a double-bottom line investing in MFIs as part of their broader development and impact mandates. It may also be useful for a broader audience concerned with micro- and agricultural finance and rural development, including international financing institutions (IFIs) and other development practitioners.This publication is part of the Investment Toolkits series under the FAO Investment Centre's Knowledge for Investment (K4I) series.