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In the course of respiratory infections, the efficacy of microbiologic diagnosis has increased years after years, in term of specificity, sensitivity and rapidity. New pathogenic agents have been described such as: Legionella pneumophila, Chlamydia pneumoniae, Hantavirus. Some viruses have been well characterized as responsible for seasonal outbreaks using rapid tools for identification. Needs for efficient diagnostic tools became more obvious when specific antiviral drugs appeared on the market. So technologic developments improved the efficacy of microbiologic diagnosis and anticipate a better specificity as well as sensitivity with the help of molecular biology. Respiratory syncytial virus is one of the major infectious agents found in respiratory infections in young children and newborns. On the whole it was detected in more than one third of pediatric nasopharyngeal aspirations received in our laboratory and more than 50% during the peak of the winter epidemics. The method of direct antigen detection by immunofluorescence with the help of monoclonal antibodies allowed us to establish an incidence curve of these recurrent outbreaks, beginning in December to stop usually by the end of April. During this same period, influenza A virus, seldom influenza B virus, were detected in many nasopharyngeal specimens. Other viruses, parainfluenza 1 to 3 and Adenovirus, were irregularly detected all along the year. In the great majority of nasopharyngeal aspirations with a positive virus detection, one virus only was observed. Antigen detection methods were also developed for some bacteria such as Chlamydia pneumoniae, Legionella pneumophila. Although serology is not frequently used by pediatricians, it is still necessary for the diagnosis of Mycoplasma pneumoniae infections. A direct antigen detection test is now available, but its sensitivity needs to be evaluated. On the other hand serologic diagnosis may be extremely useful when long lasting or treatment resistant respiratory infections occur. Seroconversion or four-fold increasing titers to one pathogen may be observed when a second serum sample is tested together with the first serum of this patient. The diagnostic yield will be all the more efficient that time between both samples is long. Molecular biology techniques will significantly change the way to investigate an infection. Presently these methods are used in research laboratories, but automated technologies will facilitate routine laboratory workload. Screening methods using multiplex PCR are also promising.