As efficient cationic photo-initiators have appeared, a number of industrially important polymers are produced by cationic photo-polymerization. Significant advances in the area of the catalysis of cationic photo-polymerization have opened the doors to many applications of cationic chemistry which previously were thought to be unattainable. Cationic polymers have unique properties, which cannot be obtained from free-radical polymerization. In this review, we investigate the general mechanism of cationic photo-polymerization and show the application in the adhesive field.