Abstract
This paper examines the formation of social organizations in the two forms of urban habitat most commonly used by low-income groups in Buenos Aires - the invasion of vacant land, mostly in peripheral areas, and the occupation of vacant buildings within the city centre. In the first, community organizations are widespread, of long standing and relatively effective. They helped develop (and negotiate for) basic infrastructrure and services and helped negotiate land tenure. Government agencies recognize them and work with them. In the second, community organizations are less evident and less effective, in part because the illegally occupied buildings are scattered between those legally owned by higher-income groups, in part because the inhabitants seek to disguise their illegal status and in part because government organizations do not recognize the legitimacy of such organizations and have made no provision to allow their tenure to be regularized.