The relationship between fatty acids and breast cancer has been debated for long, because of the high frequency of breast cancer and the contradictory results from the numerous studies devoted to this issue. The present review includes case-control and prospective studies, according to specified methodological criteria, which estimated the exposure to monounsaturated fatty acids (MUFA) and n-6 and n-3 polyunsaturated fatty acids (PUFA) using dietary questionnaires or markers (plasma, erythrocytes, adipose tissue). The relationship between MUFA intake and breast cancer risk seems to depend on the contributing food : neutral or beneficial for vegetable oil, rather deleterious for animal products. Contrary to data from animal experiments, human studies do not show an increase of breast cancer risk with n-6 PUFA intake. Estimating the risk associated with alpha-linolenic acid appears difficult due to the incompleteness of food composition tables and studies on biomarkers remain few. The same applies to long-chain n-3 PUFA despite the suggestion of a decrease in risk, in agreement with animal studies. However, it is difficult in human to disentangle the effect of nutrient intake from that of contributing foods or even nutritional profile.