Strategic decision making and evaluation in philanthropic giving and social investment requires good-quality information about the social impacts of that investment. One way to meet this need is by calculating a social return on investment (SROI) measure, akin to the return on investment (ROI) approach used in business analysis. Despite much buzz in the field, SROI measurements are rarely used, in part because of the complexity of the calculations but also because of a number of thorny and often expensive organizational challenges associated with implementing an SROI process. This article explores these implementation challenges by comparing four social venture organizations in the health care field—two in the Netherlands and two in the United States—that have utilized some sort of SROI measurement. We summarize the SROI process and identify the specific organizational challenges in each case. Lessons learned from this analysis include the value of process versus product and the importance of fitting the type of measurement to the organizational context. We conclude with a summary of best practices for organizations and social investors who might try to make effective use of SROI measure