Currently, a substantial part of clinical practice is involved with health maintenance and disease prevention. However, there continue to be deficiencies in the delivery of preventive care, and it is sometimes difficult to identify which interventions are effective--for lack of randomized clinical trials--and the theoretical basis of this approach is not always clear. In this paper, the first in a series of two, we will present the general conceptual principles of preventive medicine, starting with levels of prevention and followed by the criteria for selecting a preventive intervention. Regarding the latter, the characteristics of the disease to be screened and the respective screening test will be discussed, as well as therapy for positive cases. Finally, we will discuss the possible consequences of screening for disease. In our second paper, to be published in the next issue, we will present guidelines for the critical appraisal of an article about screening.