Acute respiratory infections are the second cause of morbidity and mortality in children and adults worldwide, being viruses, bacteria and fungi involved in their etiology. The rapid diagnosis allows for a better clinical management of the patient, for adopting public health measures and for controlling possible outbreaks. The main etiologic agents can be diagnosed within the first hours after the onset of symptoms with antigen detection techniques, primarily immunochromatography. Results are obtained in 15-30minutes, with 70-90% sensitivity and >95% specificity for the diagnosis of Streptococcus pneumoniae and Legionella pneumophila serogroup O1 infections from urine, Streptococcus pyogenes from throat swabs and respiratory syncytial virus from nasopharyngeal aspirates. Worse results are obtained for influenza viruses and Pneumocystis jirovecii with these techniques; however, other easy-to-perform molecular techniques are available for the rapid diagnosis of these microorganisms. In general, these techniques should not be used for monitoring the outcome or response to treatment.