Females and males display striking patterns of sexual dimorphism in many animals and other organisms. Differences are documented in morphology, physiology and behaviour. In recent years, advances in genomics technologies have facilitated the discovery of genetic mechanisms underlying sex-specific phenotypes, showing that sex-biased expression of hundreds to thousands of genes across the genome contributes to male and female phenotypic differences. The sex-specific evolutionary forces shaping sexual dimorphisms have also begun to be unravelled, including the interactions between ‘master’ sex determining genes on the sex chromosomes, and other genes during development to control sex-biases of other genes. This symposium will focus on the molecular basis and evolution of sexual dimorphism across animals and other organisms, including the origins, evolution and biology of sex chromosomes. It will cover topics ranging from the evolution of sex determining systems, sex linkage and sex chromosomes, sex-biased gene expression (on autosomes and sex chromosomes) and X chromosome dosage compensation mechanisms, to the evolution of associated dimorphic phenotypes and the underlying selective pressures.