Cellulose-based thin films and membranes have tremendous potential to improve the sustainability and performance of packaging, food & drugs, and energy devices. Yet, a deeper understanding of their formation and structure, in both the wet and dry state, is needed to meet application specific demands and scale-up. This work aims to develop this understanding, along with characterization methods to determine structure-property relationships of cellulose-base membranes. Specifically, by using a combination of microfibrillated cellulose, (MFC), cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) this work explores the formation and structure of all-cellulose composite membranes and the role of surface area, charge and pressure. We present novel labelling and imaging techniques to characterize membrane structure in both the wet and dry state. Dewatering times, filtration cake resistance and compressibility are measured in an effort to understand the role of intermolecular forces during membrane formation. Overall, the findings of this work not only provide insight into the formation of all-cellulose composite membranes, but provide tools and methods to characterize these materials for a variety of applications.