The STFI OPTI-Kappa is now a well established sensor for on-line measurement of the kappa number. There are sensors installed in Sweden, Japan, the US and Finland. There are substantial experiences from measurements on different types of pulps, from different wood species and different positions of installations. There are also considerable experiences from control applications in cooking, oxygen delignification and bleaching, where more uniform pulp quality and higher production efficiency have been achieved. In the article measurement and control principles are presented and experiences are summarized.