The Small Enterprise Foundation (SEF) is a micro-finance NGO working in South Africa to provide savings and credit facilities to support business development of the poorest people. Reacting to a realisation that they were not reaching the poorest people, the SEF undertook a pilot study using participatory wealth ranking to establish people's own criteria of poverty. These proved to differ from the externally judged criteria that they had been using to assess eligibility for membership, and led to the adoption of participatory mapping and wealth ranking instead. Their challenge was then one of scale; how to apply this methodology to villages of 700 - 1000 households? This article focuses on some of the challenges faced in designing a cost-effective system that would work in such large villages. It uses Bhungeni village as a case study to illustrate the application of the methodology and then goes on to discuss some of the wider issues of the relevance and use of wealth ranking in the context of a micro-finance programme.