Before new interventions are released into disease control programmes, it is essential that they are carefully evaluated in `field trials'. These may be complex and expensive undertakings, requiring the follow-up of hundreds, or thousands, of individuals, often for long periods. Descriptions of the detailed procedures and methods used in the trials that have been conducted have rarely been published. A consequence of this is that those planning such trials have fewguidelines available and little access to knowledge accumulated previously, other than their own. In this manual practical issues in trial design and conduct are discussed fully and in sufficient detail such that it may be used as a `toolbox' by field investigators. It has been compiled by a group of people with direct experience in the design, conduct, and analysis of field trials, and is based on their accumulated experience.