Emission of NOx from the fluid catalytic cracking (FCC) regenerator is increasingly controlled by various state and local regulations. The FCC regenerator poses a very challenging environment for controlling NOx. Other than NO, the high-temperature flue gas contains O-2, CO, CO2, SO2, SO2, H2O, and possibly other nitrogen or sulfur species. In this paper, we will first present a complete nitrogen balance around the fluid catalytic cracking unit by using a circulating pilot plant with continuous regeneration. We will also discuss the transformation of nitrogen species during the cracking and catalyst regeneration process, which has direct implications on the formation and reduction of NOx in the regenerator. Pilot plant or commercial data on the effect of operating conditions, cracking feedstocks, and CO combustion promoter usage on NOx emission will be discussed. With both thermodynamic analysis as well as experiments, we will show that the so-called "thermal NOx" does not contribute to the FCC regenerator NOx emission. On the basis of the understanding of the nitrogen chemistry we have obtained, we have successfully developed different catalytic NOx control technologies for the FCC regenerator. Direct NOx reduction additives and a new generation of CO combustion promoters which significantly reduced NOx emissions will be discussed. Both laboratory and commercial trial data on some of the NOx control additives as well as the mechanism for the NOx control additives will also be presented. Finaly, the future directions for NOx control will be discussed.