Biomass processing plays a pivotal role in advancing circular economy practices by transforming organic waste into valuable bio-based products and energy. Through biorefining, biomass feedstocks—including agricultural residues, forest by-products, microbial biomass, food and animal waste, and industrial waste—are converted into biofuels, biochemicals, and biomaterials, reducing environmental pollution and reliance on fossil fuels. The biorefinery concept enables efficient resource utilization via enzymatic hydrolysis, anaerobic digestion, hydrothermal carbonization, and gasification, among other technologies. Current research focuses on optimizing biomass valorization through innovative pretreatment methods, enzyme advancements, and integrated bioprocessing. However, challenges such as high processing costs and biomass recalcitrance remain. The transition from first- to second-generation biomass, combined with advancements in genetic engineering and process optimization, will drive future developments. Biomass processing is a cornerstone of sustainability, fostering resource efficiency, greenhouse gas reduction, and economic viability, ultimately supporting the global shift towards a circular bioeconomy.