My aim in this article is to discuss the formation of poblaciones callampa (shanty towns) in Chile during the second half of the twentieth century (especially between 1950 and 1970), as an expression of the right of necessity of thousands of homeless persons. I suggest that this social phenomenon is the germ of the consequent tomas de sitio (illegal encampments), where the requisites to take part were “to be poor, to have children, three sticks and a flag.” The individual right of necessity is the foundation of the formal claims of politically-organized groups looking for a permanent place to live that fulfills minimal criteria of social justice. Although the argument is focused on the specific case of the Chilean callampas, I suggest that the same logic may be applied to other cases of precarious, illegal settlements in Latin America and worldwide.