We investigate possible explanations for the large decline in U.S. teen childbearing that occurred in the twenty years following the 1991 peak. Our review of previous evidence and the results of new analyses presented here leads to the following main set of observations. First, the observed decline in teen childbearing is even more surprising given the increasing share of Hispanic teens, who have higher birth rates. Second, we find that a reduction in sexual activity and an increase in contraceptive use contributed to the decline roughly equally. Third, we are able to identify a statistically discernible impact of declining welfare benefits and expanded access to family planning services through Medicaid, but combined they can only account for 12 percent of the observed decline in teen childbearing between 1991 and 2008. We are unable to find any impact of other policies (including abstinence only or mandatory sex education) or labor market conditions. In the end we conclude that the standard factors which are claimed to be related to the rate at which teens give birth appear to explain little of the recent trend.