Studies that assess large numbers of subjects for longitudinal research, for epidemiological purposes, or for the evaluation of prevention and intervention efforts, are very costly and should be undertaken with the greatest care to ensure their success. The success of a study, apart from its scientific merit, depends largely on the ability of the researcher to plan and set up a smoothly running operation. However, the skills required for such a task are often not acquired in academic training, nor do scientific journals abound with information on the practical aspects of running a large study. This paper summarizes the experience gained in executing a longitudinal study and covers aspects of planning, hiring of staff, training and supervision of interviewers, data collection and data entry and management. The importance of the use of the computer as a management tool is stressed.