The global energy demand has been projected to rise over 28% by 2040, calling for more renewable resources such as lignocellulosic biomass to produce biofuels, and for green and efficient processing technologies. Actual limitations of processing technologies include high cost and high-energy consumption. Here, deep eutectic solvents appear as a green alternative to harmful organic solvents. For biomass pretreatment, deep eutectic solvent-based techniques require about 28% less energy and remove 40.7% of lignin. Deep eutectic solvents can be recycled up to five times. Here we review deep eutectic solvents with focus on properties, toxicity, recyclability, techno-economy and applications in biomass conversion.