Background and Objectives: Light pollution is growing as the newest form of environmental pollution. Experimental and epidemiological studies suggested that exposure to artificial light at night (ALAN), especially to blue light, disrupts the circadian rhythm, inhibits melatonin production, and increases the risk of breast cancer. We assessed the association between exposure to outdoor ALAN and breast cancer. Methods: We used data from a population-based case-control study, including 1185 incident breast cancer cases and 1282 healthy controls enrolled in 2005-2007 from two French departments- Côte d’Or and Ille-et-Vilaine. Data for outdoor ALAN exposure was obtained using calibrated images from Defense Meteorological Satellite Program (DMSP) for 1995-2006. Exposure was assessed over the 10 years prior to inclusion in the study by cross-referencing the DMSP images and the geocoded locations of residences in ArcGIS. Logistic regression was used to obtain odds ratios (OR) and 95% confidence intervals (CI) for the association, adjusting for sociodemographic, reproductive, and lifestyle-related factors, night-shift work, and urbanization. Results: The overall OR for breast cancer per 1 standard deviation increase in ALAN exposure (129.29 nW/cm2/sr) was 1.05 (95% CI: 0.95-1.17). Subgroup analyses showed slightly higher ORs in post-menopausal women (OR: 1.15; 95% CI: 0.99-1.34) and in women who never worked at night (OR: 1.08; 95% CI: 0.96-1.22). The association was stronger for HER2-positive breast tumors (OR: 1.27; 95% CI: 0.99-1.62). Conclusions: Our study did not find an association between exposure to outdoor ALAN and breast cancer. However, possible associations between outdoor exposure to ALAN and breast cancer risk in specific subgroups were indicated. This study had several strengths, including use of calibrated satellite images for outdoor ALAN assessment and consideration of a large set of potential confounders. Further large-scale studies using improved exposure assessment methods, including measurement of blue light and exposure assessment at the individual level, are required.