With the continued depletion of non-renewable energy resources, it is essential to seek new methods of harnessing clean and renewable energy. In this regard, second-generation bioethanol derived from lignocellulosic biomass has attracted increasing attention in recent years. The choice of the pretreatment method of lignocellulose is critical to the subsequent bioconversion processes. Compared with other conventional chemical pretreatment methods, hydrothermal pretreatment is a simple, low-cost, and economically feasible process that requires water as the only reagent. This paper reviews the research efforts that have been made toward hydrothermal pretreatment of lignocellulosic biomass and focuses on the transformations involving cellulose, hemicellulose, and lignin during this process.