This study compares four different techniques for measuring the crystallinity index (CI) of cellulose, including X-ray diffraction (XRD) and solid-state 13C nuclear magnetic resonance (NMR). The simplest method, which involves measuring just two heights in the X-ray diffractogram, produced significantly higher crystallinity values compared to other methods. The authors argue that alternative XRD and NMR methods, which consider the contributions from both amorphous and crystalline cellulose, provide a more accurate measure of crystallinity. Despite the high amorphous content often being more easily digestible by enzymes, the study suggests that CI does not clearly indicate the digestibility of a cellulose sample. The complex nature of cellulose structure and the influence of factors such as lignin, hemicellulose content, porosity, and particle size on enzymatic hydrolysis are discussed. The authors caution against correlating small changes in CI with changes in cellulose digestibility due to the methodological dependencies and the complexity of cellulase interactions with amorphous and crystalline celluloses.This study compares four different techniques for measuring the crystallinity index (CI) of cellulose, including X-ray diffraction (XRD) and solid-state 13C nuclear magnetic resonance (NMR). The simplest method, which involves measuring just two heights in the X-ray diffractogram, produced significantly higher crystallinity values compared to other methods. The authors argue that alternative XRD and NMR methods, which consider the contributions from both amorphous and crystalline cellulose, provide a more accurate measure of crystallinity. Despite the high amorphous content often being more easily digestible by enzymes, the study suggests that CI does not clearly indicate the digestibility of a cellulose sample. The complex nature of cellulose structure and the influence of factors such as lignin, hemicellulose content, porosity, and particle size on enzymatic hydrolysis are discussed. The authors caution against correlating small changes in CI with changes in cellulose digestibility due to the methodological dependencies and the complexity of cellulase interactions with amorphous and crystalline celluloses.