Breast is a typical female sexual physiologic organ that is influenced by steroid hormone from menarche until menopause. Therefore various diseases can be developed by continuous action of estrogen and progesterone. Breast diseases are mainly categorized as benign and malignant. It is very important to distinguish the malignancy from breast diseases. However, it is very difficult to diagnose malignancy in pregnant and lactating women even though the same breast diseases took place. Therefore, we will review breast diseases such as breast carcinoma during pregnancy and lactation.