Now more than 50 years after interferon was discovered, in 1957, by Isaacs and Lindenmann, this quinquagenarian has evidently come to age. Its major indications for clinical use are, for interferon-alpha, now mostly used in its pegylated form, the treatment of chronic hepatitis B and chronic hepatitis C, the latter generally in combination with ribavirin; and for interferon-beta, the treatment of multiple sclerosis (MS). Of quintessence in the 50 years with interferon has been the cloning of interferon-alpha and -beta in 1980, which not only proved its identity, but also laid the basis for its application in the treatment of hepatitis (B and C) and multiple sclerosis, respectively.