Current studies on the groundwater ages in Mt. Fuji are reviewed. Mt. Fuji is one of the largest Quaternary stratovolcanoes in Japan (volume of 1,200-1,500 km3). The large amount of precipitation on mountain slopes (annual volume of approximately 2 × 109 m3) suggests that Mt. Fuji contains substantial reservoirs of groundwater in its main body. In fact, numerous springs located around the foot of the mountain originate mainly from confined groundwater in Holocene lava flows. Early groundwater studies in the Mt. Fuji area focused on the development of groundwater resources, followed by studies on measures to address groundwater problems including depletion, salinization, and nitrate contamination. Application of isotope hydrological tools since the 1990s has provided valuable information on groundwater flow processes in Mt. Fuji. Groundwater age in Mt. Fuji has been a key issue since the 1960s, and relatively extensive data on tritium (3H) are available. Besides, new age-dating techniques including tritiogenic 3He (3H/3He method), chlorofluorocarbons (CFCs), and bomb-produced 36Cl have been applied in the Mt. Fuji area in recent years. These groundwater age data are compiled and discussed in terms of the hydrogeological structure of Mt. Fuji (lava flows of the Younger Fuji volcano, and mudflow deposits of the Older Fuji volcano). Compiled multi-tracer groundwater age data show distinct differences between Younger Fuji (< 30-40 years) and Older Fuji (> 60 years) aquifers, although data on Older Fuji groundwater are still limited. Possible explanations relate to differences in permeability or volume between Younger Fuji and Older Fuji deposits.