Besant’s 1859 study of their behavior began the long series of studies of under-water bubbles. Although microbubbles are known to erode the propellers of oceangoing ships and degrade the performance of fluid machinery, they are useful in medical, marine, chemical, and a wide array of other industrial fields. Their useflness is explained by their properties of flexibility and their ability to produce vibrations, which make them an excellent secondary acoustic source, as well as ability ultrahigh temperatures and pressures. Here we discuss the mechanism of the cavitation phenomenon, the history of microbubble research, and the RPNNP equation, which, in turn, forms the basis of bubble dynamics, which led to the establishment of bubbleology. We also discuss ultrasound contrast agents, which greatly enhance the value of ultrasonic imaging, subharmonic imaging using contrast agents, and ultrasonic drug delivery systems, and point out directions indicated by recent research. We further venture from the field of medicine itself and report findings on single-bubble sonoluminescence, which has attracted attention because of its potential value in cold fusion, and discuss studies in which a microjet induces erosion in a fluid.