Ultrasound is a promising technology, which has been used in many mass transfer unit operations with excellent results, such as drying, osmotic dehydration, extraction, hydration, salting/brining, and desalting. During a mass transfer unit operation, external and internal resistances can exist. Then, the reduction of both is necessary to accelerate the process. Ultrasound can reduce one or both resistances by different mechanisms such as acoustic cavitation, the sponge effect, microchannel formation, and others. Furthermore, the ultrasound enhancement differs in each unit operation. Consequently, the use of this technology for assisting the entire process or as a pretreatment can change the way the process is improved.