This CDI Practice Paper by Pauline Oosterhoff, Sowmyaa Bharadwaj, Danny Burns, Aruna Mohan Raj, Rituu B. Nanda and Pradeep Narayanan reflects on the use of participatory statistics to assess the impact of interventions to eradicate slavery and bonded labour. It deals with: (1) the challenges of estimating changes in the magnitude of various forms of slavery; (2) the potential of combining participatory approaches with statistical principles to generate robust data for assessing impact of slavery eradication; and (3) the practical and ethical questions in relation to working with people living within a context of modern slavery. The paper draws lessons from the realities of using participatory statistics to support the evaluation of a slavery eradication programme in North India.