BACKGROUND: A few studies suggest that air pollution may decrease fertility, but prospective studies and examinations of windows of susceptibility remain unclear. OBJECTIVE: We aimed to examine the association between time-varying exposure to nitrogen dioxide (NO2), ozone (O3), fine particulate matter <2:5 lm (PM2/5), and black carbon (BC) on in vitro fertilization (IVF) outcomes. METHODS: We included 345 women (522 IVF cycles) for the NO2,O3, and PM2/5 analyses and 339 women (512 IVF cycles) for the BC analysis enrolled in a prospective cohort at a Boston fertility center (2004–2015). We used validated spatiotemporal models to estimate daily residential exposure to NO2,O3,PM2/5, and BC. Multivariable discrete time Cox proportional hazards models with four periods [ovarian stimulation (OS), oocyte retrieval to embryo transfer (ET), ET to implantation, implantation to live birth] estimated odds ratios (OR) and 95% confidence intervals (CI) of failing at IVF. Time-dependent interactions were used to identify vulnerable periods. RESULTS: An interquartile range (IQR) increase in NO2,PM2/5, and BC throughout the IVF cycle was associated with an elevated odds of failing at IVF prior to live birth (OR = 1:08, 95% CI: 0.95, 1.23 for NO2;OR=1:06, 95% CI: 0.88, 1.28 for PM2/5; and OR = 1:16, 95% CI: 0.96, 1.41 for BC). This relationship significantly varied across the IVF cycle such that the association with higher exposure to air pollution during OS was strongest for early IVF failures. An IQR increase in NO2,PM2/5, and BC exposure during OS was associated with 1.42 (95% CI: 1.20, 1.69), 1.26 (95% CI: 0.96, 1.67), and 1.23 (95% CI: 0.96, 1.59) times the odds of failing prior to oocyte retrieval, and 1.32 (95% CI: 1.13, 1.54), 1.27 (95% CI: 0.98, 1.65), and 1.32 (95% CI: 1.10, 1.59) times the odds of failing prior to ET. CONCLUSION: Increased exposure to traffic-related pollutants was associated with higher odds of early IVF failure. https://doi.org/10.1289/EHP4601.