The development of second-generation bioenergy crops holds great promise for sustainable energy production. Efficient conversion of plant biomass into biofuels relies on the digestibility of the cell wall, which constitutes a major barrier to the release of fermentable sugars. This book chapter aims to provide a comprehensive understanding of cell wall digestibility, diversity, and complexity in the context of developing second-generation bioenergy crops. The chapter begins by highlighting the significance of second-generation bioenergy crops and the crucial role of cell wall digestibility in bioenergy production. It then delves into the composition and structure of the cell wall, discussing the major components such as cellulose, hemicellulose, and lignin, as well as the structural organization of the cell wall. Factors affecting cell wall digestibility are explored, including genetic factors, plant breeding strategies, and environmental influences that impact cell wall composition. Additionally, pretreatment methods for enhancing cell wall digestibility are discussed. Analytical techniques for assessing cell wall digestibility are presented, including chemical analysis methods, enzymatic assays, and imaging techniques for visualizing cell wall structure. The chapter emphasizes the importance of these techniques in evaluating the effectiveness of bioenergy crop development efforts. The diversity and complexity of cell wall structures are examined, encompassing variations in cell wall composition among different plant species and genetic and phenotypic diversity within plant populations. The implications of cell wall diversity for bioenergy crop development are discussed, highlighting the potential for harnessing natural diversity to improve digestibility. Finally, the chapter explores engineering strategies for enhancing cell wall digestibility, encompassing genetic modification approaches, breeding for improved cell wall traits, and biotechnological methods. Future perspectives, challenges, and sustainability considerations in bioenergy crop development are also addressed. In a nut’s shell, this book chapter provides a comprehensive overview of cell wall digestibility, diversity, and complexity in developing second-generation bioenergy crops. The insights presented here contribute to the advancement of bioenergy technologies by offering a deeper understanding of cell wall properties and their implications for sustainable energy production.