Air pollution has been related to brain structural alterations, but a relationship with white matter microstructure is unclear.We assessed whether pregnancy and childhood exposures to air pollution are related to white matter microstructure in preadolescents.We used data of 2,954 children from the Generation R Study, a population-based birth cohort from Rotterdam, Netherlands (2002-2006). Concentrations of 17 air pollutants including nitrogen oxides (), particulate matter (PM), and components of PM were estimated at participants' homes during pregnancy and childhood using land-use regression models. Diffusion tensor images were obtained at child's 9-12 years of age, and fractional anisotropy (FA) and mean diffusivity (MD) were computed. We performed linear regressions adjusting for socioeconomic and lifestyle characteristics. Single-pollutant analyses were followed by multipollutant analyses using the Deletion/Substitution/Addition (DSA) algorithm.In the single-pollutant analyses, higher concentrations of several air pollutants during pregnancy or childhood were associated with significantly lower FA or higher MD (). In multipollutant models of pregnancy exposures selected by DSA, higher concentration of fine particles was associated with significantly lower FA [ (95% CI: , ) per fine particles] and higher concentration of elemental silicon with significantly higher MD [0.06 (95% CI: 0.01, 0.11) per silicon]. Multipollutant models of childhood exposures selected by DSA indicated significant associations of with FA [ (95% CI: , ) per increase], and of elemental zinc and the oxidative potential of PM with MD [0.03 (95% CI: 0.01, 0.04) per zinc increase and 0.07 (95% CI: 0.00, 0.44) per oxidative potential increase]. Mutually adjusted models of significant exposures during pregnancy and childhood indicated significant associations of silicon during pregnancy, and zinc during childhood, with MD.Exposure in pregnancy and childhood to air pollutants from tailpipe and non-tailpipe emissions were associated with lower FA and higher MD in white matter of preadolescents. https://doi.org/10.1289/EHP4709.