Impact investments have the potential to play an important role in solving social and environmental problems. Although the sector is growing rapidly, it does face a number of challenges, in particular related to impact measurement. Measuring the impact of such investments, which aim to achieve social and/or environmental impact while simultaneously generating financial returns, has proven difficult. This study examines the design and application of measurement systems related to impact investments. To investigate this, the seven impact measurement guidelines of the IMWG are used as a framework. We study to which degree impact investors set concrete investment objectives, how they measure and collect data related to the generated impact of the investments, and how they use such data to evaluate investment opportunities. We rely on a qualitative research methodology, including 13 semi-structured interviews among Dutch institutional investors. We find that impact investors typically set general, but not specific impact objectives. Furthermore, we note that impact investors are still searching for and experimenting with performance measures, and that they would value the development of standardized measures. Such standardized measures may assist in reducing the cost of obtaining investment data, while simultaneously increasing data reliability. Although the obtained impact data is currently hardly used for external reporting and impact data driven investment decisions, the institutional investors expect this to happen in the near future as the process of impact measurement matures. This would enable institutional investors to transition from performance measurement to performance management in the impact investment industry.