Vitamin D is an essential nutrient in the human diet. A unique property of vitamin D is that it can be produced by endogenous synthesis in the skin following sufficient Ultraviolet B (UVB) radiation. In fact, our understanding of this compound has changed, such that it is no longer consider a true vitamin, but rather a steroid hormone. De-identified data for this analysis were derived from women residing in Jeddah, Saudi Arabia who completed routine medical visits in the summer of 2009 at King Fahad Hospital (KFH). In Chapter 1,“THE ASSOCIATION BETWEEN VITAMIN D STATUS IN NORMAL WEIGHT VERSUS OBESE WOMEN RESIDING IN WESTERN SAUDI ARABIA” we evaluate the relationship between body size and serum 25(OH)D concentrations including the association between change in body size during adulthood and vitamin D status. This study examines whether the current weight and weight change since age 18 years are associated with vitamin D status. This study found that neither current weight nor adult weight gain were associated with vitamin D status in Saudi Arabian women. In chapter 2,“IS AVOIDING SUN EXPOSURE VIA SUN PROTECTION PRACTICES ASSOCIATED WITH LOW VITAMIN D STATUS IN SAUDI ARABIAN WOMEN?” we investigate whether women who avoid UV exposure have lower 25(OH)D concentrations than women who do not avoid exposure. UV exposure was defined by time in outdoor activities, use of protective clothing and sunscreen. This study demonstrated that avoiding UV exposure via indoor activity and the use of sunscreen or/and wearing protective clothing was not associated with vitamin D status. Chapter 3, “VITAMIN D STATUS AND BREAST CANCER IN SAUDI ARABIAN WOMEN: A CASE CONTROL STUDY” we examine if vitamin D status as assessed by serum concentrations of 25(OH)D would be lower in breast cancer cases as compared to controls. This study demonstrated that there is a significant relationship between higher serum concentrations of 25(OH)D and lower risk of breast cancer. Chapter 4, “IMPLICATIONS AND FUTURE DIRECTIONS” is presented a summary of key findings from the three studies in this dissertation to determine avenues of further research. The appendices consist of materials related to the dissertation work.