Community-acquired pneumonia is severe when hospitalisation is required. The diagnosis is based on pulmonary involvement (chest radiography) associated with vital signs abnormalities (mental status, temperature, blood pressure and ventilatory rate) or immunosuppression (neoplastic diseases). No specific causative pathogen could be incriminated. S. pneumoniae is the main causative pathogen. Prognosis-related factors are numerous. Among them, the effectiveness of the initial antimicrobial therapy appears as the main factor directly related to the medical intervention. First-line antimicrobial therapy, instituted as soon as possible, is empirical and must, always, be effective against S. pneumoniae. For pneumonia requiring admission into ICU, a wider antimicrobial spectrum based on antimicrobial combination is used. Mortality rates varying from 3 to 30% could be early predicted by specific prognostic indices.