Objectives-This report assesses the contributions of the changing maternal age distribution and maternal age-specific infant mortality rates on overall and race and Hispanic origin-specific infant mortality rates in the United States from 2000 to 2017. Methods-The analyses used 2000-2017 linked birth and infant death data from the National Vital Statistics System. Age-adjusted infant mortality rates, based on the 2000 U.S. maternal age distribution, were calculated for each year. These rates were compared with crude rates for all births and for specific race and Hispanic-origin groups. Decomposition analysis was used to estimate the proportion of the decline due to changes in maternal age distribution and in age-specific mortality rates. Results-During 2000-2017, the age of women giving birth rose as infant mortality rates declined, although unevenly across maternal age groups. The maternal age-adjusted infant mortality rate in 2017 was 6.13 compared with the crude rate of 5.79, resulting in a 0.34 percentage point difference. Changes in the maternal age distribution accounted for 31.3% of the decline in infant mortality rates for all births and for births to non-Hispanic white women, and for 4.8% of the decline in births to non-Hispanic black women. Declines in age-specific mortality rates accounted for the remainder of the decline for these groups and for all of the decline in births to Hispanic women. Conclusion-Changes in the age distribution of women giving birth accounted for about one-third of the decline in infant mortality rates from 2000 through 2017; declines in maternal age-specific mortality rates accounted for about two-thirds of this decline. These patterns varied by race and Hispanic origin.