The present state-of-the-art of high-power semiconductor lasers and their application is reviewed. Three phenomena which prevent semiconductor lasers from emitting high power, i.e. kink, roll-over and COD(catastrophic optical damage) in power-current characteristics are discussed and the means to solve are introduced. The two important problems, the removal of heat generated in lasers and collimation methods of diverging light emitted from lasers are described. The recent trends of application such as laser processing (cutting, welding, forming), pumping of solid state lasers, optical memory and display are reviewed.