The paper gives results of studies of the applicability and effectiveness of fuel/air staging (variously called In-furnace NOx Reduction, reburning, or biased firing) in controlling nitrogen oxide (NOx) emissions from refinery process heaters and cement kilns. The studies were conducted in three laboratory subscale furnaces and during a short-term test on a full-scale cement kiln. Test results showed that 80-85% reductions in stack NOx emissions can be achieved by biased firing and the optimum fuel split is around 50%. It was also found that reductions in NOx were slightly more effective with a retrofit lance approach.