The background of the cover picture shows a schematic representation of a hydrogenase, the topic of this cluster issue, as a watermark. Superimposed are illustrations that highlight the scope of the articles: [NiFeSe] hydrogenases (bottom right-hand corner and background) are the focus of the Microreview by Inês Pereira and Pedro Matias et al. (p. 948 ff). Seigo Shima et al.(p. 963 ff) examine the iron guanylylpyridinol cofactor of [Fe] hydrogenase (bottom left-hand corner). From a theoretical point of view, Claudio Greco and Alexey Silakov et al. (p. 1043 ff) probe the magnetic properties of [FeFe]-hydrogenases (top right-hand corner). The use of hydrogenases in vivo and as hybrids for H2 production (top left-hand corner), as well as of cobalt and nickel functional models for H2 oxidation and production (bottom middle) are covered in the Microreviews by Erwin Reisner (p. 1005 ff) and Morris Bullock et al. (p. 1017 ff), respectively. Many thanks to the authors for their creative designs.