The premises, questions and methodological problems of a multicentre study aimed at describing the epidemiology of patients' problems that require a nursing intervention are described. The main points and methodological challenges are presented and discussed: (a.) the choice of focusing on patients cared for in Nursing homes and districts because a. they are complex, originate very high nursing workload, b. few studies and data from the nursing point of view are produced; c. information on everyday care and problems are lacking, d. they are marginal populations both in research and organization of care; (b.) the need of involving nurses in a surveillance exercise, to increase the attention on problems, their causes and potential avoidability, focusing the attention on problems related to drugs and devices; (c.) the importance-and challenge- of describing what happens in everyday' care, taking in account the variability related to contexts and people. The recognition of a problem is, in fact, related to factors such as the knowledge of the patient and his/her story, the level of knowledge and expertise of the nurse, the attention paid, the context and the organization. The main lessons learned are summarised.