BACKGROUND: Ambient air pollution exposure has been associated with childhood asthma, but previous studies have primarily focused on prevalence of asthma and asthma-related outcomes and urban traffic-related exposures. OBJECTIVE: We examined nationwide associations between pre-and postnatal exposure to ambient air pollution components and asthma incidence in children age 0–19 y. METHODS: Asthma incidence was identified from hospital admission, emergency room, and outpatient contacts among all live-born singletons born in Denmark between 1998 and 2016. We linked registry data with monthly mean concentrations of particulate matter (PM) with aerodynamic diameter ≤2:5 lm (PM_2:5) and PM with aerodynamic diameter ≤10 lm (PM₁₀ ), nitrogen dioxide (NO₂), nitrogen oxides, elemental carbon, and organic carbon (OC), sulfur dioxide, ozone, sulfate, nitrate, ammonium, secondary organic aerosols, and sea salt. Associations were estimated with Cox propor-tional hazard models using fixed prenatal exposure means and time-varying postnatal exposures. RESULTS: Of the 1,060,154 children included, 6.1% had asthma during the mean follow-up period of 8.8 y. The risk of asthma increased with increas-ing prenatal exposure to all pollutants except for O₃ and sea salt. We also observed increased risk after restriction to asthma after age 4 y, after addi-tional adjustment for area-specific socioeconomic status, and for postnatal exposure to most pollutants. The hazard ratio (HR) associated with an interquartile range increase of 2.4 and 8:7 lg=m³ in prenatal exposure was 1.06 [95% confidence interval (CI): 1.04, 1.08] for PM_2:5 and 1.04 (95% CI: 1.02, 1.05) for NO₂, respectively. This association with PM_2:5 was stable after adjustment for NO₂, whereas it attenuated for NO₂ to 1.01 (95% CI: 0.99, 1.03) after adjustment for PM_2:5. For a 0:5-lg=m³ increase in prenatal OC exposure, for which biomass is an important source, the HR was 1.08 (95% CI: 1.06, 1.10), irrespective of adjustment for PM_2:5. DISCUSSION: These findings suggest that early-life exposure to ambient air pollution from multiple sources contributes to asthma development. https:// doi.org/10.1289/EHP11539.