This review summarizes the results of studies on the effects of environment on breast cancer risk. As known risk factors such as reproductive life, inheritance, and socioeconomic status are estimated to explain only about half of the breast cancer cases, it has been thought that environmental factors could also be related to the risk of this disease. It is known that ionizing radiation is an environmental risk factor increasing the risk of breast cancer. The data of experimental studies show that some organochlorines could be associated with breast cancer risk although the data from epidemiological studies are not consistent due to the difficulties to assess exposure and other risk factors. Recent experimental studies show that cadmium is an environmental factor that mimics the effects of estradiol in estrogen-responsive breast cancer cell lines while solar radiation possibly decreases the risk due to protective effect of vitamin D. The data on the effect of electromagnetic fields are not consistent. Although evidence about the effect of environmental factors on the risk of breast cancer is not convincing, some of these factors together with inheritance, reproductive life, and age at exposure could be associated with an increased risk of the disease.