Economic literature has focused on both causes and effects of trade union membership; however, most of the studies about trade unions in Colombia have been focused on its effects over wages and inequality. On this document we present an estimation of the structural determinants of trade union membership, including some specific features as region/sector effects, using the Integrated Household Survey (GEIH) 2007. We find that trade union membership in Colombia seems to depend on the same structural determinants found by Johnson (2005) for a sample of countries. Nonetheless, as trade union activists have been specific victims of homicides, we argue that determinants of trade union memberships in Colombia are more complex than their Latin American counterparts.