This chapter delves into the essential aspects of second-generation (2G) biofuels, highlighting the innovative conversion technologies and primary feedstocks that position them as sustainable alternatives to fossil fuels and first-generation biofuels. Unlike first-generation biofuels, which are derived from food crops such as corn and sugarcane, 2G biofuels utilize nonfood biomass. This includes a variety of sources, such as corn stover, sugarcane bagasse and vinasse, forest residues, vegetable oils, and animal fats that are not suitable for food consumption. This significant distinction not only mitigates the competition with food production but also addresses concerns related to food security and land use. Furthermore, the chapter outlines the key types of 2G biofuels, each with unique production processes and applications. These include cellulosic bioethanol, which is produced through the fermentation of lignocellulosic materials; biodiesel derived from nonedible oils, such as waste cooking oil; biomethane, which is generated through anaerobic digestion of organic matter; and biohydrogen, created from various biomass sources through thermochemical or biological processes. Each of these biofuels plays a crucial role in advancing sustainable energy solutions, reducing greenhouse gas emissions, and supporting the transition to a more circular economy.