Cellulose nanocrystals (CNCs) are bio-based, needle-shaped nanoparticles – this chapter describes their production, characterization, and applications. CNCs are isolated from natural cellulose through acid hydrolysis or oxidative processes where their nano-scale dimensions give rise to impressive properties such as high mechanical strength, shear-thinning rheological behavior, and the ability to stabilize interfaces and self-assemble into liquid crystalline phases. Since CNCs behave very differently from their molecular and macroscopic counterparts, extensive characterization is required for research and development – nanoparticle size, morphology, and surface charge are key properties of interest. The promising biocompatibility and sustainability of CNCs has led to their use in emulsions, foams, gels, photonic materials, and nanocomposites for potential applications in food, cosmetics, adhesives, coatings, industrial processing fluids, biomedicine, and automotive parts, to name just a few.