This chapter aims to answer the questions: (a) Why do we need to measure migration? and (b) What should we expect from good data on migration? It analyses two data collection tools in detail: IOM’s Displacement Tracking Matrix (DTM) and the Mixed Migration Monitoring Mechanism initiative (4Mi). First, it describes and compares both tools. Both have made available to the public considerable information on mobility in countries where there was little or no previous knowledge about the topic. It thereafter focuses on DTM and combines different series of data. Two sorts of combinations are tested: data of the same nature obtained at different dates, and data of different nature obtained at the same date. It concludes by giving recommendations on how to further DTM’s data collection efforts.