‪This article analyses the emergence of the slum category in colonial India and its deployment, initially in a hygienist perspective to measure overcrowding and poor housing, and later as a means to measure the phenomenon. Two contrasting cases –the metropolises of Delhi and Madras/Chennai– are used to illustrate this evolution. After Independence, the “slum” became primarily a legal-administrative category, linked to a registration process granting rights to its inhabitants. Public statistical institutions did not apply this category until later, and it is only since 2001 that slum areas have been distinguished in the general population census. However, due to a lack of coordination, adequate assessment of slum dwellings is held back by the multiplicity of different approaches.‪