Technical Abstract: Fermentable sugar production from lignocellulosic biomass has become an important research focus in the production of renewable biofuels and other bio-products. It means conversion of the carbohydrates contained in the biomass, including cellulose, hemicellose, and/or pectin into their component sugars through chemical and biochemical methods. The hydrolysis of cellulose is one of critical steps in the conversion. Acid hydrolysis such as concentrated sulfuric acid and diluted sulfuric acid hydrolysis is a more established method than enzymatic hydrolysis. However, the enzymatic hydrolysis method is currently more desirable because it offers several advantages including lower energy requirements, minimal byproduct formation, mild operating conditions, and fewer negative impacts on the environment. The main challenges associated with enzymatic hydrolysis are the low sugar yield and high cost of the hydrolysis process. Considerable research has been conducted to improve the enzymatic hydrolysis of cellulose. Pretreatment of biomass to remove lignin/hemicellulose and modify the structure of biomass can significantly enhance the hydrolysis of cellulose. This chapter provides an overview on the sciences and technologies for producing fermentable sugars from lignocellulosic biomass, including the features of lignocellulosic biomass, pretreatment, and hydrolysis technologies. The current status of conversion technologies, existing barriers to the current technologies, and the directions for the future research are also discussed.