The educational distribution of U.S. immigrants shifted significantly to the right in recent decades as the share without a high school diploma fell and the share with a bachelor's degree rose. This improvement coincided with a shift in immigrants' origins toward Asia and rising global education levels. This study examines how much of the change in immigrants' educational distribution over 2000-2019 is due to changes in their distribution across origin countries versus rising attainment among immigrants within origin countries. We demonstrate that within-country changes account for most of the observed increase in the educational distribution. In contrast, changes in where immigrants originated played a minimal role. Finally, we show that economic conditions in origin countries can explain little of this rise, whereas demographic trends and the skill composition of U.S. temporary worker visas are significantly related to changes in immigrants' educational distribution.