The etiology of CAP includes bacteria (S. pneumoniae, H. influenzae, S. aureus, S. pyogenes, gram negative bacilli), respiratory viruses (influenza, parainfluenza, RSV, human metapneumovirus, adenovirus, coronavirus), the atypical pneumonia pathogens (Mycoplasma pneumonia, Chlamydophilia pneumonia, Legionella sp) and tropical pathogens (scrub typhus, leptospirosis, melioidosis). The percentage contribution of viruses reduces as age advances and the relative contribution of mycoplasma increases. Gram negative bacteria are mainly seen in newborns and in the elderly. Worldwide including India, S. pneumoniae (3-51%) is the commonest organism followed by H. influenza (5-21%), Mycoplasma pneumoniae (4-24%) and respiratory viruses (influenza, parainfluenza, RSV, coronavirus, human metapneumovirus 10-36%). Other agents include Legionella species (1-6%) S. aureus (1-2%) and in some predisposed individuals Klebsiella pneumoniae and Pseudomonas aeruginosa. Mycobacterium tuberculosis should also be considered a possible etiology in some individuals with a slightly protracted illness.