Iron–sulphur (Fe–S) clusters have long been recognized as essential and versatile cofactors of proteins involved in catalysis, electron transport and sensing of ambient conditions. Despite the relative simplicity of Fe–S clusters in terms of structure and composition, their synthesis and assembly into apoproteins is a highly complex and coordinated process in living cells. Different biogenesis machineries in both bacteria and eukaryotes have been discovered that assist Fe–S-protein maturation according to uniform biosynthetic principles. The importance of Fe–S proteins for life is documented by an increasing number of diseases linked to these components and their biogenesis.