This thesis looks into the development of a generic evaluation tool for Social Impact Bonds (SIBs). As a relatively new concept, SIBs are bringing change on important aspects of several sectors. The global financial crisis has not only exposed weaknesses in the financial sector, but also once again put the spotlight on the inequality that is existent in many parts of the world. Since, there has been growing interest in sustainability and realising social impact through many ways. This also brings up additional complexities though. Questions as, who is paying the bill and how do we measure social impact that has been realised become more important than ever, but are also much harder to answer than the comparison of the profits of traditional companies. SIBs provide a solution of bringing stakeholders from both the public and private sector to not only achieve, but also scale the social impact they deliver. So what are they? This thesis takes the definition that has been stated by Gustafsson et al: a Social Impact Bond is a model in which private investors put up capital to fund a social intervention and governments repay the investors only if an agreed-upon outcome is achieved [33][p.4]. It then goes through a design process to design, test and redesign an evaluation tool for Social Impact Bonds.