Ultrafine bubbles (UFBs) are gas-filled bubbles with a diameter smaller than 1 μm. They are sometimes called bulk nanobubbles because these are not on a solid surface but inside a bulk liquid (water). They are already being used in commercial processes such as cleaning and plant cultivation. However, many mysteries still exist with respect to UFBs, such as mechanisms of stability, OH radical formation, and biological and medical effects. This is the first book on UFBs that reviews research done on them. It is helpful for those interested in the fundamentals of this emerging field and its applications, including cleaning, biological, medical, and dental students and researchers.