Although classical influenza is a clinically typical illness ("unchanging disease due to a changing agent"), laboratory investigations are essential at the beginning of each influenza epidemic. They should confirm suspected influenza cases and exclude "flu-like illnesses" which may be caused by numerous other viral and bacterial agents. Different virological as well as serological methods are available. For early diagnosis of acute influenza virus infections, virus detection using rapid procedures for virus isolation or antigen staining and molecular biological techniques have been developed. The determination of specific antibodies (IgG, IgM) has traditionally been widely used diagnostically. Conventional serological diagnosis is possible by means of the complement fixation and hemagglutination inhibition tests and allows the detection of type- and subtype-specific antibodies, respectively. As part of an automated serology, immunofluorescence test and enzyme-linked immunosorbent assay are the mostly widely available methods. In comparison, virus detection is clearly superior to antibody determination for diagnosis of influenza virus infections. However, antibody testing may be useful as a complementary tool to confirm the diagnosis retrospectively.