Cellulose, the most abundant biopolymer on Earth, offers tremendous potential for sustainable materials and bio-based products. Understanding the interplay between processing parameters and the cellulose fiber structure is vital for optimizing manufacturing processes and advancing sustainable materials science. This thesis presents a comprehensive investigation into the effects of dynamictimescales and the magnitude of pressure on cellulose fibres’ hierarchical structureand properties, encompassing a range of experimental techniques andmethodologies. The initial study focuses on the dynamic compression and decompression of wet cellulose pulp fibers using shock waves, aiming to modify the cell wall structure significantly. By subjecting wet cellulose pulp samples to rapid high pressure pulses (10 ms long, 300 MPa peak pressure), we hypothesize that we observe increased accessibility to the fibre interior by inducing weak spots in the cell wall. Characterization techniques, including scanning electron microscopyand X-ray diffraction, reveal a decrease in crystallinity and changes in surfacemorphology, indicating structural modifications induced by the rapid pressurepulse. Subsequently, we delve into a detailed investigation of pressure induced changes to the cellulose fiber structure using in-situ and ex-situ X-ray diffraction techniques with a resistive-heated diamond anvil cell (DAC). During the in-situ experiment, we track crystalline changes in real-time during static extreme-pressure conditions, providing insights into the kinetics of cellulose transformation under pressure. In the ex-situ experiment, we examine post decompression properties to assess the retention of structural modifications induced by pressure. Additional bulk measurements using various characterizationtechniques corroborate the findings, confirming the structural changesobserved in situ. Finally, we explore a novel approach involving slow compression and fast decompression, reminiscent of a steam explosion technique, to induce destructive changes in cellulose fiber structure. This study, albeit destructive, provides valuable insights into the limits of cellulose modification under extreme processing conditions, offering perspectives on the feasibility and implications of such approaches.In conclusion, this thesis presents a comprehensive examination of pressure induced modifications in cellulose fiber structure, highlighting the critical role of processing parameters in regulating cellulose properties. By elucidating the intricate relationship between pressure application timescales, magnitudes, and structural outcomes, we pave the way for developing tailored cellulose-based materials with enhanced functionalities and sustainability.