The main features of four-neutrino 3+1 and 2+2 mixing schemes are reviewed,
after a discussion on the necessity of at least four massive neutrinos if the
solar, atmospheric and LSND anomalies are due to neutrino oscillations.
Complete list of references on four-neutrino mixing at
http://www.to.infn.it/~giunti/neutrinoComment: 7 pages. Talk presented at NOW 2000, Conca Specchiulla (Otranto,
Italy), 9-16 Sep. 200