Females and males often differ extensively in their physical traits. This sexual dimorphism is largely caused by differences in gene expression. Recent advances in genomics, such as RNA sequencing (RNA-seq), have revealed the nature and extent of sex-biased gene expression in diverse species. Here we highlight new findings regarding the causes of sex-biased expression, including sexual antagonism and incomplete dosage compensation. We also discuss how sex-biased expression can accelerate the evolution of sex-linked genes.