In this chapter, an updated overview on the applications of ultrasound (US) technology (as pretreatment or during the extraction process) to recover valuable phenolic compounds from agro-food by-products and wastes is presented. The principle of sonication, its effects on vegetal matrix and extraction mechanisms provoked by US application, the types of equipment used for ultrasound-assisted extraction (UAE), as well as the advantages and the limits of this technology, especially regarding green extraction principles, are presented. Parameters influencing UAE efficiency, as well as modeling and optimization of the process, are also discussed. Then we focus on the application of UAE for polyphenols recovery from various by-products and wastes. Aspects related to the safety and environmental issues and scale-up of UAE process are also presented. Finally, future trends for application of US technology for valorization of agro-food wastes by extracting valuable phenolics are discussed.