Most of the people now living with HIV acquired the infection through heterosexual intercourse. HIV transmission has been facilitated by (a) concomitant sexually transmitted diseases (STDs), (b) the presence of social conditions that create core groups who have frequent and numerous partners, (c) sexual practices associated with bleeding (i.e., trauma, sex during menses) as well as noncircumcision, (d) cervical ectopy, and (e) anal sex. HIV may be found both cell-free and as intracellular virus in genital tract secretion, and may be sexually transmitted through either mechanism. HIV titers in genital tract secretions vary by several logs between people and within individuals over time, being greatest just after seroconversion and with advanced immunosuppression, concomitant genital tract inflammation (including STDs), and decreasing (but not to zero) with antiretroviral therapy. The per-contact transmission efficiency rate is highly variable, ranging from > 3% to < 1 per thousand contacts, with male-to-female HIV transmission generally being more efficient than vice versa. Control of the heterosexual HIV epidemic will necessitate a multidisciplinary approach, utilizing direct biological approaches (e.g., culturally specific and behavioral interventions, as well as more fundamental community changes that decrease societal norms that augment unsafe practices.