Iron–sulfur clusters are important biological cofactors that are used for electron transfer and also for reactivity. Though the iron atoms in these clusters are typically a mixture of ferrous iron(II) and ferric iron(III), there have been reports of biological iron–sulfur clusters in which all the iron atoms are reduced to the iron(II) oxidation state. These reports have inspired synthetic studies on all-ferrous iron–sulfur clusters. This chapter describes both the biological and synthetic work on all-iron(II) clusters, drawing comparisons and noting promising avenues for future research.