The etiological diagnosis of pneumonia is necessary because it will condition therapy. The broad spectrum of potential pathogens is reduced when the host's condition and the events around the pneumonia episode are taken in account. Community acquired pneumonia in an immunocompetent host most often is caused by bacteria (predominantly S. pneumoniae) or by mycoplasma or respiratory viruses. Microbiological diagnosis relies on direct sputum examination and culture (for bacterial agents) and on serology (for nonbacterial agents). In a compromised host, the spectrum of etiological agents is broader; sputum examination often is unrewarding and invasive investigations are necessary. Bronchoscopy with bronchoalveolar lavage is increasingly used, allowing an abundant material to be analyzed with a battery of tests directed against the pathogens most probable in view of the clinical setting. Direct examinations with special stains for bacteria, fungi, parasites, and viruses offer a rapid diagnosis in some cases. Various cultural procedures for bacteria, viruses and fungi, particularly in the absence of previous antimicrobial therapy, will establish the etiological diagnosis in the majority of pneumonia cases and help to select specific therapy.