Book excerpt: The Social Impact Fund Rotterdam represents an interesting financial innovation: place-based impact investing in close cooperation with public and private partners. Place-based impact investing refers to impact investing that is focused on one particular city or region, with the advantages of being a local investor, which include better risk assessment, networks, and “boots on the ground”. These advantages should help overcome some of the problems associated with standard impact investing. This article explores how this form of place based impact investing has come about, how it works, and how it deals with the challenges of impact investing, such as effectiveness, measurement, the balance between financial and impact returns, and the allocation of societal costs and benefits. It also highlights the need for social aggregator funds: funds that exclusively invest in a portfolio of small impact investors, to create an aggregated portfolio of social impact investments that has sufficient scale, track record, and risk-return profile to be investable to large institutional investors.