This article aims to present the use of some statistical and administrative sources to investigate the immigration. In particular we will compare two sources of data: Register of residents and Census. According to the reliability of sources, the author reports which "category" of foreign people (regular immigrant, illegal immigrant, resident, long resident, etc.) or which aspects of the study object are detected by these sources. He also proposes a list of valid indicators on the level of the immigrants’ social inclusion.
Keywords: Sources of administrative data, sources of statistical data, immigration, Register of residents, Census of population, indicators.