The catalytic synthesis of ammonia developed in Germany in the 1908 to 1913 period has probably been studied more extensively during the past 50 years than any other catalytic reaction. Research endeavors include the development of a method for measuring the surface area of the catalysts; a study of the adsorption of both hydrogen and nitrogen by the iron catalysts; a phase-rule study of the formation of nitrides of iron from ammonia-hydrogen mixtures; a study of the distribution of promoters on the catalyst surface; methods for estimating the fraction of adsorbed nitrogen present in the molecular form; and estimates from infrared spectroscopy, from LEED measurements, and from ion-emission spectroscopy as to the path followed during synthesis and the catalyst planes important in the synthesis. Recently, revolutionary nonferrous catalysts severalfold more active than standard iron catalysts have been prepared by Ozaki. These throw new light on the mechanism of the catalytic synthesis of ammonia.