Until recently, the mammary duct had not been directly observed in vivo. Starting with the success of Teboul et al., studies of mammary ductoscopy (MD) for nipple discharge have been performed in Japan and other East Asian countries. Ductal lavage screening trials for breast cancer started in the 2000s. Concurrently, the number of English-language articles about MD increased. Sixty-nine English-language and 74 Japanese-language papers published in the last 19 years were reviewed. Important reports and studies were analyzed. MD has undergone significant technological development, and studies of MD have taken place in many countries. As a result, endoscopic images of the mammary duct have developed, and the endoscopic diagnosis for nipple discharge has become possible. MD-guided biopsy and surgery have been studied. Findings of MD are useful for diagnosing intraductal lesions with nipple discharge. As a result, MD has reduced the number and extent of microdochectomies. MD is also helpful in guiding breast-conserving surgery. Many pioneers have tried direct biopsy or interventions under MD, but further developments are necessary for its practical use.