First-generation ethanol plants were designed based on corn (maize grain), a dense, stable, storable and shippable commodity-type product with multiple applications. With these properties, corn was used as a feedstock for large-scale biorefineries without any challenges for a considerable length of time to allow its usage to grow exponentially. In the second-generation biofuels, the feedstocks used are low-cost carbon resources such as crop and forest residues and municipal solid waste. These materials are not dense; they have irregular size and shape, variable moisture, and are not readily storable and shippable. When the industry tested these feedstocks for biofuel production, they faced flowability, storage, transportation, and conversion issues. One way to overcome feeding, handling, transportation, and variable moisture challenges is to densify the biomass. The densification systems such as pellet mill and briquette press are commonly used to produce densified products. The densified products have uniform size, shape, and higher density. Also, the densified products are aerobically stable as they have the moisture of less than 10 % (w.b.).This book's focus is to understand how the densification process variables, biomass types and their blends, mechanical preprocessing, and thermal and chemical pretreatment methods impact the quality of the densified products produced for biofuel production.