Fishes exhibit remarkably diverse and plastic sexual development. This includes functional adult sex change, which has evolved repeatedly across the teleost tree of life. While the sex change process and its evolutionary advantages are well known, how environmental cues drive such dramatic changes in sexual identity, and the molecular processes involved, have been longstanding questions. Female-male sex change (protogyny) is common in the wrasses (Labridae), where individuals reproduce first as females, but routinely reverse sex in the absence of a socially dominant male. This process involves complete gonadal restructuring where no identifiable male tissues exist in the gonad prior to sex change. Using social manipulations to generate a sex change time-series in each of two distantly-related wrasses - the Caribbean bluehead (Thalassoma bifasciatum) and New Zealand spotty (Notolabrus celiodotus) – together with transcriptomic and whole-methylome approaches, Erica and colleagues have zeroed in on the primary trigger and subsequent molecular cascade that transforms female into male. In this talk, Erica discusses the extensive transcriptional and epigenetic reprogramming that occurs in gonadal cells during sex change, and highlights both conserved and derived aspects of the molecular network that orchestrates sex change in these two species. Neofunctionalisation of duplicated sex-pathway genes, epigenetic reprogramming and the stress axis appear to be key molecular components of sex change, yet the proximal molecular trigger that initiates the ovary-testis transformation may be an important point of diversification between species. Understanding the genomic basis of sex change in fish has important implications for understanding the evolution and functioning of vertebrate sex determination and developmental systems more broadly.