Sexual dimorphism is common throughout the animal kingdom. However, a molecular understanding of how sex-specific traits develop and evolve has been elusive. Recently, substantial progress has been made in elucidating how diverse sex-determination systems are integrated into developmental gene networks. One common theme from these studies is that sex-limited traits and gene expression are produced by the combined action of transcriptional effectors of sex-determination pathways and other transcription factors on target gene cis-regulatory elements. Sex-specific traits evolve by the gain, loss or modification of linkages in the genetic networks regulated by sex-determination transcription factors.