Introduction Gonadal origin, eggs and sperm; Chromosomes and heterogamety; Prevalence of gonochorism; Escape from Muller¿s ratchet; Multiple parentage and genetic diversity; Prelude Cytogenetics and Sex Chromosomes Chromosomes and genomes; Cytogenetics; Heterogamety Hybridization and Sex Ratio Extent of hybridizations; Hybrid identification; Viability and fertility; Introgressive hybridization; Sex ratio Gynogenesis and Consequences Gynogenics types; Purity of gynogenics; Survival and gamety; Fertility and F1 sex ratio; Sex ratio and departures Androgenesis and Autosomes Production methods; Survival; Fertility; Overriding autosomes Triploidy and sterility Types of triploids; Survival and sex ratio; Male¿s fertilizability; Female sterility Tetraploidy and polyploidy Survey and history; Scope for polyploidization; Tetraploid types; True induced tetraploids; Polyploidy; Intraploid hybridizations; Unreduced "gamete bank" Allogenesis and xenogenesis Primordial Germ Cells (PGCs); PGC transplantation system; PGCs in hybrid embryos; PGCs in heterospecific embryos; Spermatogonial Stem Cells (SSCs); Renewal of SSCs; SSC transplantation system; Relevance of PGCs and SSCs; Surgical gonadectomy Sex changers and hermaphroditism Extant of hermaphroditism; Sex change in gonochores; Sex allocation; Sex changing age; Genetic or environment? Unisexualism and reproductive modes Mutational meltdown; Exploration of reproductive modes; Squalius alburnoides; Males in unisexuals Genetic sex determination Molecular markers; Sex chromosomes; Sexonomics; Sex changers; Polygenic sex determination; Concluding remarks.