Basically, teen pregnancy rates can decrease in one of two ways—if teens have less sex or become more effective contraceptive users—or through some combination of the two. The evidence clearly indicates that more and better contraceptive use has been the main factor driving the long-term decline in teen pregnancy. The evidence, however, is much murkier when it comes to deciphering the social, cultural and economic factors affecting teens’ sexual behaviors and contraceptive use patterns. Deconstructing why teen pregnancy rates have fallen over the last several decades nonetheless matters, so that future programs, policies and practices can be shaped to help advance—rather than hinder—these positive trends.