The incidence of breast cancer (BC) is rising worldwide, with an increase in aggressive neoplasias in young women. Traditionally, BC in young women has been thought to be etiologically driven, primarily by genetic/hereditary factors. However, these factors explain only a small proportion of BCs, pointing to a role of the environment. Suspected factors responsible for this increase include lifestyle changes, notably alcohol consumption, diet with high intake of refined carbohydrates and saturated fat and low intake of polyunsaturated fatty acids (PUFA), fiber and vitamins (such as folate, vitamin D, and carotenoids), low physical activity, and body fatness, all of which may act from childhood and adolescent years through adulthood. Despite limited data on BC in young women, evidence points to the importance of a healthy lifestyle, including high intake of vegetables, fruits, legumes, fish, and poultry, low intake of sugar and fat, daily physical activity, low alcohol consumption, steady weight, and breastfeeding in preventing BC in young women. Preventive efforts should begin in early life to provide important benefits much later in life by shifting the long-term trajectory of risk accumulation. Data from Latin America and developing regions are still sparse. There is a need to harmonize studies in a global effort to fight the rise of BC incidence in low- and middle-income countries, where the nutritional transition is occurring rapidly. The stratification of BC by specific tumor characteristics needs to be considered since risk factors may be more particularly associated with the promotion of, or protection from, a defined type of BC.