Enough data are now available on the neurobiology of the avian song system and on the development and performance of song that sophisticated questions on the relations between the behavior and the neurobiology can be addressed. This review describes what is known of sex differences and individual differences in the neurobiology of the song system in mature birds. It summarizes data on the role of steroid hormones in the development of the song system and what is known of steroid-related adult plasticity in this system. Finally, it discusses hypotheses on the relations between structure and function in this system and suggests issues that must be addressed in future studies.