This chapter forges ahead with detailed discussions on migration data. The major focus is on census, as population census data across the world act as the primary source of information on migration trends. An overview of global census data and relevance over time also forms a bulk of this chapter. In the case of India, census is supplemented by large-scale surveys by the National Sample Survey Office (NSSO) which complements in understanding further the determinants of migratory patterns. This aspect is also briefly touched upon in this chapter, and further discussion entails in the next chapter.