(1) Influenza viruses and in particular sialidase (neuraminidase, EC 3.2.1.18) activity found in influenza A and B viruses, as well as 0-acetylesterase (EC 3.1.1.53) activity, characterized in influenza C virus, are an interesting research issue owing to their peculiarities and incidence in health problems. After mentioning of some recent books and reviews on both enzymes of influenza viruses, we summarize our own results on the sialidase and 0-acetylesterase of influenza viruses. (2) The release of sialidase from influenza A virus both by the proteolytic agent bromelain and the detergent /V-lauroylsarcosine results in free enzyme, which shows certain kinetic characteristics that differ in the resulting samples. Additionally, after electrofocusing, only 20% of the original activity is lost. (3) Sialidase activity can be determined by several procedures. Measurement of NADH (which is the final product of three coupled reactions, the first one depending on the sialidase action) can be performed by spectrofluorimetry, which is more sensitive than spectrophotometry. Moreover, such sensitivity is strongly increased when the measurement is carried out by luminescence. Both spectrofluorimetric and luminescent procedures allow the determination of sialidase activity either with sialidase samples released from the virion or directly with the purified virus.