Cellulose nanofibrils (CNFs) serve as one of the most promising biobased building blocks for the production of advanced sustainable materials. Extracted from lignocellulosic biomass, CNFs are highly crystalline fibrils with high aspect ratio, strength and stiffness. They can be assembled into foams, transparent films or fibrous reinforcements. [1] However, understanding and controlling these assemblies remain key challenges for their more widespread use. This research investigates the incorporation of tailorable polymeric nanoparticles into CNF networks as a strategy to control assemblies and thus bulk material properties. Polymeric core-shell nanoparticles have emerged as a highly promising category of additives for tailoring the properties of CNF-based materials. [2, 3] These nanoparticles are synthesized through polymerization-induced self-assembly (PISA), [4] mediated by reversible addition-fragmentation chain-transfer (RAFT) polymerization, producing stable aqueous dispersions of polymeric nanoparticles with tunable surface functionality, hydrophobic core and morphology. These aqueous additives can be easily hybridized with CNFs in water, providing a straight-forward strategy for modification. In this work, we investigate the influence of nanoparticle surface functionality, charge and size on bulk material properties and material structure. Material structure on different length scales is quantitively described through wide- and small-angle X-ray scattering (WAXS and SAXS), extending understanding of structure-property relationships in CNF-based materials. Our findings demonstrate that polymeric nanoparticle additives offer a versatile approach to tailoring CNF assemblies, enabling control over structural and mechanical properties. This provides valuable insights for the development of next-generation sustainable materials with enhanced performance and functionality.