There is a growing interest on ultrasonic processing of dairy products, especially fermented dairy products which are a basis to functional foods. The studies have shown that power ultrasound can enhance the fermentation process of lactic acid bacteria by modifying their metabolic activity while reducing fermentation time and improving the quality characteristics of fermented milk products. Fermentation is one of the important stages in the processing of dairy products, but it is also one of the most time and resource consuming stages during production. Thus, the benefits of ultrasound to the fermentation process due to microbial activation become increasingly important. In fact, ultrasound applications have the dual effect on microorganisms. Besides being used for microbial activation in dairy industry, it can also be used for inactivation of microorganisms depending on ultrasound power and frequency, sonication time, microorganism type, pH, and temperature. This review article summarizes the effect of power ultrasound on microbial inactivation and microbial growth based on fermentation profile of dairy products, with a theoretical background on ultrasound, including research findings. Also, the details on the activation and inactivation mechanisms of power ultrasound to microorganisms are presented.