Static and dynamic properties of nitrogen on Ru surfaces are reviewed. On Ru(0001), N occupies the h.c.p. threefold-hollow site as demonstrated by scanning tunneling microscopy and low-energy electron diffraction. For Ru(10 " 10) and (11 " 21) models for ordered structures are discussed. There is general agreement that the dissociative adsorption of N 2 is the rate-determining step in the ammonia synthesis over Ru surfaces. The dissociation of N 2 is an activated, surprisingly slow process. Different groups have observed a sticking coefficient of 10 ยฑ ยฑ12 . By density functional theory the geometry and energies of the dissociation pathway have been identified. Very recently it has been shown that the activation barrier found in experiments can only be modeled assuming the dissociation to take place at monoatomic steps. Experiments and microkinetic modeling of N 2 dissociation and NH 3 formation on Ru/Al 2 O 3 , Ru/MgO, and CsยฑRu/MgO model catalysts are discussed. Differences between the different catalysts and the single-crystal surfaces cannot be explained from the existing single-crystal results. It seems that a detailed knowledge of the morphology of the 2 nm large Ru metal particles on the different catalyst supports is needed to further develop a detailed understanding of ammonia synthesis over Ru-based catalysts.