Monosex refers to the culture of either all-male or all-female populations, a sought after approach in aquaculture. This chapter reviews the advantages of monosex population culture and details the mechanisms to achieve it based on different modes of sex determination and sexual differentiation. A recent case study for an aquaculture biotechnology based on sexual manipulation in crustaceans serves in this chapter to identify the key elements for a successful application. This application which makes use of RNA interference with a key regulating hormone opens the pathway toward environmentally friendly applications in fish and additional aquacultured species. This chapter portrays the state of the art in sexual manipulations in aquacultured species, starting with vertebrate species, followed by the case study of the crustacean species and discussion on how the techniques used in this study are applicable for other species.