An overwhelming body of clinical evidence has firmly established the HIV Undetectable = Untransmittable (U=U) concept as scientifically sound. U=U means that people living with HIV who achieve and maintain an undetectable viral load — the amount of HIV in the blood — by taking and adhering to antiretroviral therapy (ART) as prescribed cannot sexually transmit the virus to others. NIAID officials review the scientific evidence underlying U=U and discuss implications of widespread acceptance of the message.