Influenza infections have important socio-economic consequences. Risk groups identified so far include small children and elderly adults with comorbidities. In recent years in addition to seasonal influenza an outbreak of avian influenza occurred in 2005 and the new H1N1 pandemic occurred in 2009. For the latter other at risk groups were affected and a different clinical course has been documented. The focus of this article is to give an overview on the epidemiology, clinical characteristics, diagnosis and therapy of influenza infections.