Sex determination in mammals results in two discrete sexes, male and female. The sexes are genetically distinct at fertilization (XY = male and XX = female). However, there is little evidence for differences in their development until mid-gestation when the gonadal primordium forms. Recent research suggests that signals within this tissue maintain the gonad in a bipotential state, balanced between two alternative fates, testis or ovary. At mid-gestation, expression of the Y-linked gene Sry in gonadal cells triggers the divergence of gonad development toward the testis pathway. The critical role of Sry may be simply to promote the stable expression of SOX9 in the key cell population in the gonad. Establishment of SOX9 in this lineage is opposed by female promoting factors that push the gonad toward an ovarian fate. Both the male and female sides of these antagonistic pathways are augmented by feedback loops and reinforcing signals that canalize development, once the initial choice is made. Hormones and growth factors produced by the developing testis regulate the male differentiation of the sex ducts and the external genitalia during fetal life. In contrast, the ovary is not required for the fetal development of female genital ducts or female external genitalia, as these organs develop in the absence of a gonad. At puberty, hormones produced by the testis or the ovary control the sex-specific differentiation of the musculature, mammary tissue, and body hair. Male or female development of the brain was previously thought to depend on hormones produced by the testis or ovary. However, recent evidence reveals expression differences between XX and XY brains prior to the time that hormones are circulating, suggesting that some influences on brain development may be autonomous to the cells of the brain.