The environmental pollutant and common sunscreen compound oxybenzone is a benzophenone type UV light chemical filter used in industrial and consumer goods. This chemical widely contaminates human tissues, non-human species, and environmental matrices. In this dissertation, oxybenzone is investigated for its effects on the mouse mammary gland in the offspring following perinatal exposure; after perinatal and prepubertal exposures as a dual environmental insult during two sensitive times of development; and in adults after exposure during pregnancy & lactation as an environmental factor potentially increasing the tissue susceptibility to mammary tumors. Chapter 1 introduces the mammary gland. Chapter 2 reviews UV screening chemicals with a focus on oxybenzone. Chapter 3 summarizes methods used in this dissertation. Chapters 4 and 5 present two aspects of our studies quantifying the impact of perinatal exposure to oxybenzone on the epithelial and stromal compartments of the mammary gland. Chapter 6 evaluates the effects of exposures during two sensitive periods of development: the perinatal and prepubertal periods. Chapters 7 and 8 present a concise summary of breast cancer, its trends, its genetic and environmental causes, and findings on tumors acquired in BALB/c mice transplanted with p53-/- epithelium and exposed to oxybenzone during pregnancy and lactation. Chapter 9 takes an ethics-based approach to modern-day pollutants, including endocrine disrupting chemicals, in the process of decision-making towards sustainable environmental and human health. Finally, chapter 10 summarizes and hints on future directions for oxybenzone research. The results of this work make a case for oxybenzone’s disruption of the mammary gland’s epithelial and stromal compartments during several vulnerable periods of development. Our data also support an argument that animals genetically susceptible to breast cancer may have heightened sensitivity to environmental chemicals. Lastly, we propose six principles of environmental health that are imperative in decision-making on synthetic compounds. The data presented herein contribute to the body of evidence postulating that oxybenzone is an endocrine disruptor, and that developmental exposures to everyday synthetic chemicals used for human safety and/or convenience elicit later-in-life effects on susceptibility to chemical environmental exposures, on health and disease.