Ionic liquids (ILs) are environmentally benign �green� solvents that received tremendous attention due to their unique beneficial properties such as negligible vapor pressure, multiple solvation interactions with organic and inorganic compounds, excellent chemical and thermal stability, and a large electrochemical window. For more than a decade, ILs have been increasingly exploited as solvents and/or (co)solvents and/or reagents in a wide range of applications, including biomass pretreatment for further processing. ILs were found to be very effective in dissolution of biomass, even lignocellulosic biomass, at ambient temperature. The performance of enzymatic hydrolysis of biomass for the production of biofuels and bioenergy could be increased significantly via IL pretreatment. The aim of this chapter is to discuss the pretreatment processes followed by the factors that affect the efficiency of the pretreatment as well as the environmental impact of ILs. The recent progress on the pretreatment of lignocellulosic biomass and microalgae biomass using ILs is also reviewed with a particular focus on the advancement during the past few years. Finally, the environmental impacts of various ILs have been discussed followed by their recovery methods. © 2019 Elsevier B.V. All rights reserved.