The purpose of this article is to set the groundwork of data-driven decision making. Currently, there are widespread discussions on how society is shaped and changing due to the increased use of data for decision making in the private and public sector. Central to this form of decision making is big data and open data. We present a critical review of big data: it’s characteristics and sources. We also provide a critical review of open data by delineating its difference to big data, i.e. the types and sources of open data. We argue that if the conditions are right, big data can be open data—and vice versa. Most importantly, we present where and how big data can be used applied in various areas of society, e.g. in smart cities. By carrying out this review, we outline the composition of data and where and how it can be applied in society at large. Ultimately, given the accessibility of data, we critically review a fast-moving ecosystem where end-users and decision makers can be guided by data.