Sex determination, the mechanism by which embryos are committed to develop as either males or females, has been the subject of intense research during the last two decades. Gonadal development, which is a crucial event of sex differentiation, involves complex cellular processes, including the differentiation of several cell types, cell proliferation and migration as well as vascularization, which take place differently in male and female embryos. This special issue of Sexual Development presents a series of 12 articles describing the state of the art of the control of gonad development in mammals and other vertebrates. Several of the most renowned international experts in the field of sexual development review the genetic, hormonal and environmental operators controlling gonad development in mammals, birds, reptiles, amphibians, fish and nematodes, as well as the cases of sex reversal and gonad abnormalities derived from mutations in the human and other mammals.Providing both basic and advanced insights into the control of gonad development in a variety of animal taxa, this issue is essential reading for both students and researchers in the fields of genetics, developmental and reproductive biology, zoology and evolutionary biology.