The International Migration database presents data showcasing flows and stocks of the total immigrant population and immigrant labour force; together with data on acquisition of nationality.

There are three sources for the data: national statistics offices; Eurostat and the United Nations.

Four types of sources are used: population registers; residence permits; labour force surveys and censuses. In countries that have a population register and in those that use residence permit data; stocks and flows of immigrants are most often calculated using the same source. There are exceptions; however; as some countries instead use census or labour force survey data to estimate the stock of the immigrant population. In studying stocks and flows; the same problems are encountered whether population register or permit data are used (in particular; the risk of underestimation when minors are registered on the permit of one of the parents or if the migrants are not required to have permits because of a free movement agreement). To this must be added the difficulty of purging the files regularly to eliminate permits that have expired.