The aim of this review is to focus on the epidemiology of lower respiratory tract infections, the etiology, prognosis and risk factors, dividing these problems into the following issues: global impact of these afflictions, community-acquired pneumonia, hospital acquired pneumonia, respiratory infections in surgery, acute bronchitis and exacerbations of chronic bronchitis. Every year about 5 million people die of acute respiratory infections. Among these, pneumonia represents the most frequent cause of mortality, hospitalization and medical consultation. Several factors (age, underlying disease, environment) influence mortality, morbidity and also microbial etiology. The authors also refer to recent data on the most frequently identified antibiotic resistance of respiratory pathogens. The knowledge of such different clinico-epidemiological situations is essential to physicians for an effective approach to treatment of pneumonia and bronchitis.