With the advancements in nanotechnology, cellulose nanofibers (CNFs) have gained substantial importance due to their nanoscale dimension, ease of preparation, tuneable surface properties, high aspect ratio, and high surface area. However, understanding the extraction procedure and sources used to produce CNFs is crucial to maximizing their potential for biodegradable packaging, water treatment, biomedical, and electronic applications. CNFs can be obtained from plant-derived fibers that are further fibrillated from microsized to nano-level through mechanical methods with pretreatment. Preparation of nanofibers from abundantly available renewable materials such as agricultural waste and crop residues offers a promising alternative to fossil-based polymers. This approach fosters and facilitates the production of a wide range of materials with improved properties. This chapter provides a comprehensive review of the potential of CNFs, preparation methods, pretreatment approaches, and emerging applications. It highlights the role of CNF as a new eco-friendly material derived from sustainable cellulose sources, suitable for diverse industrial sectors. As a crucial component in sustainable materials, CNFs drive innovation while advancing industrial nanomaterials toward sustainability.