This chapter discusses the complexity and diversity of biodegradable waste, which is the main part of the global waste stream. For efficient management of biodegradable waste—arising from municipal solid waste, agricultural residues, food processing by-products, wastewater sludge, green waste, and industrial organic waste—the various components that it encompasses must be understood well. This chapter identifies the types of biodegradable waste and factors affecting their composition: geographical, socioeconomic, and environmental conditions. Due to significant variability in waste composition, there is a need for a systematic approach to the characterization of wastes. Several methods involving physical, chemical, and biological analyses of key properties of biodegradable waste are detailed to optimize biorefinery design. Waste characterization not only helps in selecting appropriate feedstocks but also improves process efficiency, product quality, and environmental impact assessment. Characterization provides insights into the energy and nutrient potential of different waste streams, which supports the selection of appropriate technologies and conditions for resource recovery and product development. More so, waste characterization data also come in handy with the environment for sustainability because it would create a strategy of reduction in greenhouse emissions, energy use, and consequently less waste creation. That said, the bottom line is that waste characterization will help change green biorefinery, converting garbage waste into gold for resource sustainability.