This paper discusses the effects of ultrasound on the production of particles by precipitation and crystallization. Examples are given from the formation of crystals of BaSO4, K2SO4, TiO2 and sucrose. It is shown that ultrasound reduces the induction time, narrows the width of the metastable zone and leads to the production of more, finer, and more uniform crystals in some cases. The reasons for these effects of ultrasound on the nucleation of crystals are discussed and a possible mechanism is presented.