Conventional wisdom is that charities are required to keep their money in two silos: for investment or for grant-making. It is often assumed that trustees should maximise returns by investing only in mainstream financial products without regard to their match or mismatch with their charitable purposes. Social investment challenges this model. Over the last thirty years or so, practitioners in the UK and US have engaged in a range of social investment activities - defined as investments which generate a social as well as a financial return. This briefing and the full report describe the principles and concepts behind social investment and use case studies - from both the UK and the US - to help shed light on how social investments have actually happened and worked in practice.