The diagnosis of community acquired pneumonia (CAP) is based on a patient history with respiratory symptoms and additional symptoms and signs such as fever over more than 4 days, dyspnea and tachypnea and/or a positive lung auscultation. Despite recently developed tests, radiology is a key diagnostic procedure for confirming CAP. Importantly, the first treating physician must judge whether to hospitalize a patient or not. Two major scoring systems allow judgement of severity and short-term prognosis. In general, in patients with mild or moderate pneumonia who can be treated on an ambulatory basis, no specific microbiological diagnosis must be performed. If, for clinical or epidemiological reasons a gram stain is done, it must be obtained from purulent sputum. Recent tests may help in discriminating between viral and bacterial pneumonia (procalcitonin test) or determine the bacteria responsible for acute disease (pneumococcal antigen test using urine).