A major objective of the Prevention of Blindness programme of the World Health Organisation (WHO PBL Programme) is that by the year 1989 National Prevention of Blindness Programmes should be established in 60 developing countries which have a huge excess of avoidable blindness. To date, this has been achieved in 57 developing countries. Centres in a number of academic institutions have been designated "WHO Collaborating Centres for Prevention of Blindness". These are actively supported by the WHO PBL Programme to undertake collaborative survey and research work in developing countries. A principal aim of such work is to generate epidemiologically sound information that could form the bases for rational planning, implementation and proper evaluation of programmes for prevention of blindness. This paper outlines the main epidemiological methods that have been employed recently, by a WHO collaborating centre based at the Institute of Ophthalmology in London, in population based eye surveys and in epidemiological research in a number of developing countries. The key aspects of the methodology are discussed in detail in the context of practical experience. The outcome of the studies are also mentioned briefly. The three distinct types of epidemiological studies discussed are: (1) Population-based sample surveys, (2) Longitudinal cohort (follow-up) studies and (3) Hospital-based case-control studies.