Fish exhibit a striking variety of sex determination mechanisms and sex chromosome structures, differing sometimes even between closely related species. Therefore fish are not only interesting objects to study the molecular mechanisms of sex determination operating in a species of interest but also provide models to better understand the evolution of this process. The review will mainly focus on one species, the medaka (Oryzias latipes), a small laboratory fish model species originating from freshwater biotopes of Japan and Korea, because the most advanced knowledge of genetic sex determination in a non-mammalian species has been obtained in this species. The master male sex-determining factor (dmrt1bY) has been isolated and identified as a duplicate of an autosomal gene that is known to function at a most downstream position of the sex-determining regulatory gene cascade from worms and flies up to mammals. In medaka, the entire male-specific region of the Y-chromosome and adjacent pseudo-autosomal regions are sequenced. The Y-chromosome is only 5-10 million years old. This allows to infer the molecular events that have shaped the medaka Y and to evaluate this against the predictions of the common theory of evolution of sex chromosomes. The molecular mechanisms how dmrt1bY initiates male development are only beginning to be understood, but it is apparent that the dmrt1bY gene functions by inhibiting male primordial germ cell proliferation at the sex-determining stage.