Cytochromes of c-type contain covalently attached hemes that are formed via thioether bonds between the vinyls of heme b and cysteines within C(1)XXC(2)H motifs of apocytochromes. In diverse organisms this post-translational modification relies on membrane-associated specific biogenesis proteins, referred to as cytochrome c maturation (Ccm) systems. A highly complex version of these systems, Ccm or System I, is found in Gram-negative bacteria, archaea and plant mitochondria. We describe emerging functional interactions between the Ccm components categorized into three conserved modules, and present a mechanistic view of the molecular basis of ubiquitous vinyl-2 approximately Cys(1) and vinyl-4 approximately Cys(2) heme b-apocytochrome thioether bonds in c-type cytochromes.