Experimental studies and models of NOx formation in simple nonpremixed flames at atmospheric pressure are reviewed. Laminar flames are briefly discussed, while the bulk of the review focuses on axisymmetric, turbulent, jet flames. The issue of the scaling of NOx emission indices with nozzle exit diameter, initial jet velocity, and fuel type is a major theme of the article. The failure of a simple leading-order scaling is investigated and interpreted in terms of the various interrelated parameters affecting NO formation : the relative importance of various NO-forming chemical pathways, departures of O-atom concentrations and temperatures from their equilibrium values, flame strain, and flame radiation.