Spent hops represent one of the significant by-products of the brewing industry with an untapped potential for valorisation. Spent hops are rich in bioactive compounds such as polyphenols, essential oils, and bitter acids, exhibiting antimicrobial, antioxidant, and nutraceutical properties. Current research highlights their potential use as functional food ingredients, natural pesticides, biofuel sources, and phytotherapeutic adjuvants. This chapter provides an analysis of the environmental impact of this by-product, its chemical composition and scientific research findings on its emerging applications in various sectors, including food, agriculture, bioenergy and pharmaceuticals, while assessing the challenges hindering its large-scale adoption. Indeed, the transition to a circular economy model in brewing could mitigate waste and unlock high-value applications for spent hops, promoting both sustainability and economic benefits.