Lipid bilayers are the main structural component of the membranes that shape and compartmentalize cells. Cell architecture is highly dynamic and membranes' conformation changes dramatically in processes such as movement, division, and vesicle trafficking. Fluidity plays essential role in the structural malleability and diversity of static shapes of membranes. However, its importance in the dynamics of membrane deformations is less appreciated. Membrane bending by thermal or active forces is commonly assumed to be damped by viscous losses in the surrounding medium. In this talk, I will present our recent experimental and theoretical work where we demonstrated that dissipation within the membrane controls the undulation dynamics of nonplanar membranes with a radius of curvature smaller than the Saffman-Delbruck length. Using flickering spectroscopy of giant vesicles made of DPPC:Cholesterol and pure diblock-copolymer bilayer membranes, the signature of membrane dissipation was detected in curvature fluctuations [1]. We extend the theoretical analysis to submicron liposomes, where lipid density fluctuations, which arise from the stretching and compression of the monolayer leaflets, and intermonolayer friction become important. The results highlight the crucial role of intramembrane dissipation in cellular membrane remodeling and in the thermally driven curvature fluctuations of submicron liposomes. [1] HA Faizi, R Granek, PM Vlahovska “Curvature fluctuations of fluid vesicles reveal hydrodynamic dissipation within the bilayer”, PNAS, 121 (44), e2413557121 (2024)