This study compares four methods for measuring the crystallinity index (CI) of cellulose using X-ray diffraction (XRD) and solid-state 13C nuclear magnetic resonance (NMR), and evaluates their impact on interpreting cellulase performance. The four methods include the XRD peak height method, XRD deconvolution method, XRD amorphous subtraction method, and NMR C4 peak separation method. The results show that the XRD peak height method produces significantly higher CI values than the other methods. Literature data for Avicel PH-101 support this observation. The alternative XRD and NMR methods, which consider the contributions from amorphous and crystalline cellulose to the entire XRD and NMR spectra, provide a more accurate measure of cellulose crystallinity. However, it is unclear whether CI actually provides a clear indication of the digestibility of a cellulose sample. Cellulose accessibility is affected by crystallinity, but is also likely to be affected by other parameters such as lignin/hemicellulose contents and distribution, porosity, and particle size. The study cautions against trying to correlate relatively small changes in CI with changes in cellulose digestibility. Additionally, the prediction of cellulase performance based on low levels of cellulose conversion may not include sufficient digestion of the crystalline component to be meaningful. The study concludes that the most popular method for estimating cellulose CI, the XRD height method, produces values that are significantly higher than the other methods. However, the other methods studied in this work rank the celluloses in roughly the same order as the XRD height method, but the CI values from the height method are significantly higher. The study suggests that the other XRD and NMR methods provide a more accurate measure of the crystallinity of cellulose samples.This study compares four methods for measuring the crystallinity index (CI) of cellulose using X-ray diffraction (XRD) and solid-state 13C nuclear magnetic resonance (NMR), and evaluates their impact on interpreting cellulase performance. The four methods include the XRD peak height method, XRD deconvolution method, XRD amorphous subtraction method, and NMR C4 peak separation method. The results show that the XRD peak height method produces significantly higher CI values than the other methods. Literature data for Avicel PH-101 support this observation. The alternative XRD and NMR methods, which consider the contributions from amorphous and crystalline cellulose to the entire XRD and NMR spectra, provide a more accurate measure of cellulose crystallinity. However, it is unclear whether CI actually provides a clear indication of the digestibility of a cellulose sample. Cellulose accessibility is affected by crystallinity, but is also likely to be affected by other parameters such as lignin/hemicellulose contents and distribution, porosity, and particle size. The study cautions against trying to correlate relatively small changes in CI with changes in cellulose digestibility. Additionally, the prediction of cellulase performance based on low levels of cellulose conversion may not include sufficient digestion of the crystalline component to be meaningful. The study concludes that the most popular method for estimating cellulose CI, the XRD height method, produces values that are significantly higher than the other methods. However, the other methods studied in this work rank the celluloses in roughly the same order as the XRD height method, but the CI values from the height method are significantly higher. The study suggests that the other XRD and NMR methods provide a more accurate measure of the crystallinity of cellulose samples.