Breast cancer is the most common malignant carcinomas for females in the world. Recently, potential viral factors for breast cancer have been emphasized, as human papillomavirus, Epstein­-Barr virus and the others were detected in breast carcinomas. There is no clear conclusion for the relationship between viruses and breast carcinogenesis for discrepancy among the data reported from different studies. The main obstacles and bottleneck of studies are analyzed in this review.