Impact investing has captured the world’s imagination. Just six years after the Rockefeller Foundation coined the term, the sector is booming. As this sector moves from the margins to the mainstream, it’s important to consider: What will it take for impact investing to reach its full potential? This question is hard to answer because, in the midst of all of this excitement, there aren’t clear success markers for the sector. Without those, the institutions managing the billions of sector dollars won’t be able accurately to assess the risks they are taking and, more important, the returns, both financial and social, they hope to generate.