Ultrafine bubbles (UFBs) are gas-filled bubbles having a diameter smaller than 1 mm. They are sometimes called bulk nanobubbles because the nanobubbles are not on a solid surface but inside a bulk liquid (water). UFBs have already been used in commercial processes such as cleaning and aquacultures. However, there are still many mysteries surrounding UFBs, such as mechanisms of stability, OH radical formation, and biological and medical effects. This is the first book on UFBs and it reviews researches done on them, which will be helpful for readers and researchers interested in the fundamentals of this emerging field and its applications, including cleaning, biological, medical, and dental application of ozone ultrafine bubble water.