The sustainable conversion of biomass waste into added-value chemicals is critical for addressing global environmental challenges and promoting the transition toward circular bioeconomy. This chapter provides an overview of chemical conversion pathways that enhance the value of biomass waste through both catalytic and non-catalytic processes. Key valorization reactions discussed include hydrolysis, transesterification, esterification, and dehydration, which enable the transformation of complex biopolymers such as cellulose, hemicellulose, and triglycerides into platform molecules, biofuels, and specific chemicals. Emphasis is placed on the role of catalytic systems including homogenous and heterogeneous catalysts in improving conversion efficiency, selectivity, and product yield.