National statistics as any other public or private record, that give an idea of foreing immigrants and their descendants in Mexico almost always offer vague numbers, which require careful scrutiny for the data to be used as reliable indicators of demographic behavior that is difficult to measure. This essay generally analyzes the possibilities and limitations of some countable sources, since the late 19th century to the present, for studying the phenomenon. The general census results from 1895 to 2000, as well as different social and demographic indicators in the national census related to the foreing population residing in Mexico are evaluated. Research also takes into account periodicals dealing with long-term migratory movement and other public and private records, which although they were not compiled for statistical purposes have been used by experts in the analysis of specific foreing groups in specific periods.