The relationship between fat intake and breast cancer has been debated for a long period of time. Animal, ecological, and case-control studies have supported that dietary fat increases breast cancer risk. However, cohort studies have not support any strong association between dietary fat intake and breast cancer risk. It has not been clear whether fat per se or some specific type of fat is responsible for the increased risk. Recently, a few cohort studies have found some positive association between specific types of fat intake, but not overall fat intake and breast cancer risk. In this review, the findings from previous studies will be summarized with advantages and disadvantages of different types of study design and recent findings will be introduced.