Firm-level micro-data have become increasingly important in OECD activities not only for econometric analysis that captures heterogeneity across enterprises but also in order to conduct different and more detailed types of data aggregation (e.g. by geographical unit, firm size, industry). This paper describes the development of the OECD ORBIS micro database. This database includes more than 200 variables providing financial and ownership information for over 44 million companies across the world. The paper provides an overview of the nature of these data, and of the advantages and disadvantages of such administrative micro-data for research. It describes the data included in the OECD ORBIS database and the cleaning procedures that have been undertaken to identify suspicious values. The paper then presents some preliminary aggregate results showing the potential of the database. Finally, it identifies some structural biases in the database and the steps that should be taken to correct them.