In Japan, research on functional water, especially on reduced water, is developing at a rapid pace. Reduced water such as electrochemically reduced water and natural reduced water can scavenge reactive oxygen species in cultured cells. Reduced waters are expected to have preventive and positive effects on oxidative stress-related diseases such as diabetes, cancer, arteriosclerosis, neurodegenerative diseases, and side effects of hemodialysis. It has been suggested that the active agents in reduced water are hydrogen (atoms and molecules), mineral nanoparticles, and mineral nanoparticle hydrides.