As the planet’s most abundant biopolymer, cellulose is the foundation of industries from textiles and construction to pharmaceuticals and biomedicine. In this chapter, we dive deep into how modern vibrational spectroscopy, particularly infrared (IR) and Raman spectroscopy, can unravel the intricate details of cellulose’s structure. We demonstrate how these tools can identify cellulose’s molecular conformations and distinguish its various crystalline forms (cellulose I, II, III, and IV). They also allow us to gauge how crystalline a sample is (the crystallinity index), reveal the orientation of cellulose fibers, and detect subtle chemical modifications in its structure. Along the way, we highlight key spectral fingerprints and employ robust analytical methods such as polarized spectroscopy for orientation analysis, crystallinity indexing for quantifying order, and ATR-FTIR to extract meaningful insights. Finally, we explore emerging trends like performing spectroscopy in situ and in real time, harnessing machine learning to enhance spectral interpretation, and combining spectroscopic techniques with other analytical tools for a more comprehensive understanding. By bridging foundational theory with real-world applications, this chapter serves as an engaging, thorough guide that provides researchers with valuable spectroscopic insights into cellulose, inspiring further exploration and innovation in cellulose-based materials.