The impetus to formulate immigration policy, recently evident in the Carter and Reagan Administrations, cornes from a growing awareness -- both public and private -- of the phenomenon of illegal migration, which nowadays has emerged as a most pressing issue. The same issue motivates the present study. The concern, however,is not with illegal migration at the national level, but rather at the local level specifically, undocumented workers in one of Houston's most powerful industries: Construction, The purpose of this study is to provide an insight into the demographic characteristics, country of origin, employment patterns, wage and working conditions, public services participation, duration of stay and ties in the U.S., in order to assess the impact of undocumented workers gainfully employed in Houston's labor market. The current study is unlike previous studies which considered only apprehended aliens. Undocumented workers in this sample were young, disadvantaged adults who came from Latin America to find employment. One out of three respondents had been in the U.S. for one or more years and their work had helped them to support at least one dependent in their homeland. Undocumented workers had significantly less education than their U.S. fellow workers and most of them had no technical training (the majority before coming to the U.S. had worked in agricultural and blue-collar activities). Most illegals were unskilled or semi-skilled workers in their most recent construction job. Undocumented workers prefer the kind of job that will allow them to earn the most money in the least possible time. By coming to Houston, the illegal worker encountered better job opportunities and less possibility of being detected by the Immigration and Naturalization Service (INS) than if they had stayed near the border or in small towns. The time of permanency for the majority of respondents is linked to their experience in avoiding detection and their ability to secure a job. Knowing about these characteristics and the illegal workers' performance in the labor market provides important leads on their impact or. U.S. society and in the formulation of a sensible immigration policy. This study propose a major emphasis on continuous cooperation with those countries which constitute the major source of undocumented workers, to improve their economies and their capacity to control their migrant's outflow.