<p>This paper analyze some of principal union´s challenges, because of the transformation of work, whose fundamental sign is the tercerization and itsconsecuences. The analysis focus in the case of an industrial district of Ambito Metropolitano de Buenos Aires. The methodology is qualitative, based in deep interviews and meeting with union´s leaders and delegates belonging to different tradition of most important metallurgical union in Argentina Unión ObreraMetalúrgica. In addition, we use secondary sources and cuantitative data.</p>