Dwindling fossil fuel stocks, rising demand for petroleum fuels from developing nations, and the burgeoning cost of conventional fossil fuels raise the danger of the global energy supply. Among various renewable energy resources, biomass usage as a sustainable biofuel feedstock represents a viable alternative to fossil fuel. Hydrothermal liquefaction (HTL) is a novel thermochemical process that aims to produce bio-crude at mild operating temperatures, reducing energy consumption and eliminating the need for preliminary drying by using a variety of high-moisture feedstocks. HTL crude bio-oil is often more stable than rapid pyrolysis oil, with lower oxygen and moisture levels, energy density, and calorific value. Solid organic wastes like municipal, industrial and agricultural waste, whose management and applications require extensive research, can be effectively utilized as feedstock for bio-crude production. Plastic as a co-reactant improves bio-oil production and decreases the environmental concerns of waste plastic landfilling, which might result in effective waste management. This chapter reviews various characterization methods of solid wastes, HTL studies on organic and solid wastes, the implication of reaction strategies, and product quantity and quality parameters. This chapter provides an extensive overview of the HTL process, including the fundamental mechanisms, the effects of operating conditions such as temperature, pressure, and residence time, and the challenges associated with feedstock variability and product characterization. Finally, the limitations and future scope for commercial HTL technology are discussed.