Unit　commitment　is　a　very　important　issue　of　generation　scheduling　in　electric　power　system.　This　kind　of　mixed-integer　programming　problem　has　been　an　active　research　subject　because　of　its　potential　cost　savings.　This　paper　presents　a　general　mathematical　model　to　the　problem,　and　compares　some　optimization-based　solution　methods　for　past　several　decades.　Some　aspects　requiring　further　study　are　also　described.