This paper, presented at the Society for Applied Anthropology Congress in Amsterdam, describes the emergence of illegal migration as a social phenomenon of world wide significance. Specifically it examines the nature of the social relations that evolve when a large number of an alien population reside legally in a foreign country and present among them are a considerable number whose status is questionable or even illegal, and whose localization and expulsion from that country is actively conducted by law enforcement authorities. Illegal persons from the Dominican Republic who reside in New York are used to illustrate how illegal migrant status operates to set up certain kinds of social relations which in turn explicate behavioral patterns, attitudes, and values, which often seem incongruous to those outside the group. Often, these behaviors frustrate and even prevent communication with institutions such as schools, welfare agencies, and law enforcement agencies, who wish to assist them. Some problems and limitations affecting traditional methodological techniques, used in social anthropology and deemed as ineffective when dealing with illegal migrants, are mentioned. (Author/AM)