This chapter explores the structural and functional characteristics of cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs), derived primarily from wood and other natural sources. Cellulose, the most abundant natural polymer, plays a critical role in various industrial applications due to its unique properties, including high tensile strength, significant aspect ratios, and favorable surface chemistry. The production methods for CNFs and CNCs are discussed, highlighting mechanical, enzymatic, and chemical treatments that enhance their properties. The chapter also examines the hierarchical structure of cellulose, the differences between bacterial cellulose and plant-derived cellulose, and the implications of nanocellulose in sustainable materials. Recent advancements in industrial-scale production and energy efficiency for nanocellulose processing are addressed, emphasizing the growing interest in these materials for diverse applications in nanotechnology and material science.