Diabetes and breast cancer are common diseases with a massive public health impact. With continual advancements in breast cancer detection and treatment diffusing into clinical practice, the population of breast cancer survivors is growing with a current estimate of 2.5 million women. Meanwhile the prevalence of type 2 diabetes continues to increase at alarming rates, largely attributed to the growing obesity epidemic. The overarching goal of this dissertation was to address current gaps in the scientific literature using data from the Women's Healthy Eating and Living (WHEL) Study- a randomized controlled trial designed to test whether a dietary pattern high in vegetables, fruit, and fiber and low in fat would reduce the risk of recurrence and all-cause mortality among women previously treated for early-stage breast cancer. The specific dissertation objectives were to: 1) assess the effects of prevalent diabetes on breast cancer disease-free survival and overall survival, 2) assess the effects of pre-cancer body mass index (BMI) and post-diagnosis weight change on the risk of incident diabetes and 3) assess the effects of physical inactivity, weight gain, and obesity on long-term worsening of glycemic control. Dissertation results showed that diabetes was independently associated with a statistically significantly higher risk of all-cause mortality in breast cancer survivors. A significant contribution was made by providing evidence suggesting that the severity or duration of diabetes may affect the risk of breast cancer recurrence and mortality. This dissertation also provides new evidence that women who experience major weight loss with subsequent regain after breast cancer are at twofold greater risk of becoming diabetic than women who maintain their pre-cancer weight. With the vast majority of breast cancer patients surviving more than 5 years beyond diagnosis, oncologists are challenged to expand their focus from acute care to managing the long-term health consequences of breast cancer. This dissertation provides compelling evidence that lifestyle changes may improve the length and quality of life of breast cancer survivors, and thus represents a significant contribution towards an issue of global public health concern.