About 5 years ago a Battelle Colloquium on “The Physical Basis of Heterogeneous Catalysis” was held in honor of Professor Emmett where he presented an introductory talk on ″Fifty Years of Progress in the Study of the Catalytic Synthesis of Ammonia″ [1], a field to which he had made significant contributions during this whole period. So why another report on this topic? Justification can perhaps partly be obtained by some of Professor Emmett's concluding remarks which were not completely decisive with respect to the mechanism of this reaction: 'The experimental work of the past 50 years leads to the conclusion that the rate-determining step in ammonia synthesis over iron catalysts is the chemisorption of nitrogen. The question as to whether the nitrogen species involved on the surface is molecular or atomic is still not conclusively resolved. In fact, our own work in this field was mainly induced by this paper, and continuing discussions and correspondence with Professor Emmett were extremely helpful during its progress. I am therefore very much honored to be this year's recipient of the Emmett Award.