Sexual development in fish is characterized by extraordinary variation, including genetic and environmental sex determination, male and female heterogamety, single gene and polygenic systems, protandry, protogyny and simultaneous hermaphroditism, social influence on sexual determination, and many combinations and variations between and within these systems. In recent years, major breakthroughs have been made in studying sexual development in fish, attracting the attention of researchers in other fields such as developmental biology, genetics, evolution, ecology, conservation and aquaculture. This special issue brings together some of the recent hot topics in this field and provides an overview of the broad range of scientific aspects that benefit from knowledge on sexual development in fish. It presents ten reviews by international experts who explore the genetic basis of sex determination, gonadal differentiation, environmental effects on sexual development, evolutionary views and application in fisheries and aquaculture. An excellent source of up-to-date information, this publication is highly recommended to researchers and students interested in the evolution of sex determination, fish genetics and breeding.