The cotyledon represents one of the bases of classification within the plant kingdom, providing the name-giving difference between dicotyledonous and monocotyledonous plants. It is also a fundamental organ and there have been many reports of cotyledon mutants in many species. The use of these mutants where they have arisen in Arabidopsis has allowed us to unravel some of the complexities of embryonic patterning and cotyledon development with a high degree of resolution. The cloning of genes involved in cotyledon development from other species, together with physiological work, has supported the hypothesis that there exists a small number of orthologous gene hierarchies, particularly those involving auxin. The time is therefore appropriate for a summary of the regulation of cotyledon development gleaned from cotyledon mutants and regulatory pathways in the model species Arabidopsis and what can be inferred from cotyledon mutants in other species. There is an enormous variation in cotyledon form and development throughout the plant kingdom and this review focuses on debates about the phylogenetic relationship between mono- and dicotyledony, discusses gymnosperm cotyledon development and pleiocotyly in natural populations, and explores the limits of homology between cotyledons and leaves.