Viral respiratory infections are very frequent diseases with variable degrees of severity. During the past few years, new respiratory viruses have been discovered by means of molecular biology techniques. In fact, these so far unidentified viruses have been present in humans for a long time but remained unidentified. These viruses are mainly represented by the human metapneumovirus (HMPV), the coronaviruses HCoV-NL63, HCoV-NH and HCoVHKU1, the human bocavirus (HBoV), and the newly described human polyomaviruses WU and KI. Beside these newly identified viruses, new respiratory viruses have emerged in humans within the last years. These viruses have been introduced from animal reservoirs and they generally present a high degree of pathogenicity with high level of mortality in humans. These emerging respiratory viruses are represented by the henipaviruses (Hendra and Nipah viruses), the New World hantaviruses associated with the Hantavirus Pulmonary Syndrome, the coronavirus responsible for the Severe Acute Respiratory Syndrome (SARS-CoV), and the avian influenza virus H5N1. This latter constitutes a major threat, with the risk of a murderous pandemic offering some parallels with the 1918-1919 Spanish flu which caused 20-50 millions deaths worldwide.