In recent years, the issue of data collection and the measurement of the impact of co-operatives has been the focus of scientific and practitioner debate. In order to understand the role and potential of co-operatives, it is necessary to realistically quantify the overall size of the sector. Despite growing interest, knowledge about the economic and social dimensions of co-operatives worldwide is still fragmentary. A realistic estimate of the economic and social impact of co-operatives is required to demonstrate that co-operatives are neither small nor marginal organizations. Evidence is needed to be able to show that a valid model exists that is different from the for-profit model dominant in today’s global economy. Developing such an estimate requires efforts to define the target population, identify the most appropriate tools for data collection and, finally, identify and define the key indicators for the assessment of co-operatives’ economic and social impact. Given these premises, this chapter will provide some considerations on the subject of data collection, presenting the importance of collecting data on co-operatives from the perspectives of the International Labour Organization (ILO) and the International Co-operative Alliance, two organizations that are increasingly involved in promoting co-operatives worldwide. In particular, we will explain why the two organizations advocate the collection of data on co-operatives, and we will present some projects advanced by these organizations, including the World Co-operative Monitor, a project promoted by the International Co-operative Alliance in conjunction with the European Research Institute on Cooperative and Social Enterprises (EURICSE).