This report presents national estimates for several types of sexual behavior among men and women aged 15–44 years in the United States in 2006–2008, as well as measures of sexual attraction and identity for adults aged 18–44. These behaviors and characteristics are relevant to birth and pregnancy rates, as well as to the incidence of sexually transmitted infections (STIs), including human immunodeficiency virus (HIV)—the virus that causes acquired immune deficiency syndrome (AIDS) (1–3). The Centers for Disease Control and Prevention (CDC) estimates that about 19 million new cases of STIs occur each year (2). About one-half of all STIs occur among persons aged 15–24, and the direct medical cost of these diseases for that age group alone was estimated at $6.5 billion in the year 2000 (4). In 2008, CDC estimated that rates of chlamydia increased, and the largest numbers of reported cases of chlamydia and gonorrhea were among teenagers aged 15–19 (5).