Special Topic Sexual Development 2015, Vol. 9, No. 1 Providing new insights into the developmental mechanisms involved in the evolution of external genitals, the articles in this special issue provide a comparative analysis across terrestrial vertebrates. The evolutionary transition from external to internal fertilization in vertebrates was facilitated by the emergence of novel organs, the external genitals. Reptiles as well as birds exhibit remarkable phenotypic diversity, ranging from the paired hemipenises of snakes and lizards, to the elaborate corkscrew penises of waterfowl, and to the complete absence of an intromittent penis in most birds. This special topic issue presents new studies on the external genital development in birds, lizards, snakes, alligators, and turtles. The articles discuss new insights into the evolutionary developmental biology (‘evo-devo’) of external genitals and provide explanations for the evolution of human congenital malformations, such as hypospadias. Offering novel perspectives on the evolution of reproductive organs, Evolution of External Genital Development will be of special interest to developmental and reproductive biologists as well as comparative and evolutionary morphologists.