The increasingly complex health care systems, together with more vulnerable, highly informed and demanding patients, conform a clinical environment in where adverse effects (AE) related to health care practice appear. The incidence of AE in hospitalized patients has been estimated between a 4 and a 17%. Twenty-five per cent of them were serious and half were considered avoidable. Seventy per cent of the AE are due to technical failures, faults in the decision making process, inappropriate performance based on the available information, problems in the anamnesis, and absent or inadequate health care provision. The explanatory model of the causal chain of an adverse effect supports that systems failures are more important than people failures. The IDEA Project seeks to study the incidence of AE related to health care for the first time in Spain. To facilitate the necessary change from a punitive culture to a proactive culture, a multidisciplinary approach of the problem taking into account the point of view of health professionals, patients, community leaders and courts is needed.