Nanocomposites are suitable for advanced engineering applications, i.e., automotive, aeronautics, biomedical applications, catalysts, gas-separation membranes, contact lenses, bioactive implant materials, bone applications, and food packaging due to their higher mechanical and thermal resistance properties. Environmental friendly natural fibre-based bionanocomposites are important because of their renewable, biodegradable and compostable nature. Natural fibers both in nano, i.e., cellulose nanocrystal (CNC), microfibrallated cellulose (MFC), microcrystalline (MCC), and macro-sized, i.e., carboxymethyl cellulose (CMC) are used to produce bionanocomposites together with bio or synthetic polymers, i.e., starch, polyurethane (PU), polylactic acid (PLA), using different methods. These composites have already proved to be invaluable gifts to the present and future generations in many different aspects, and thus, thanks go to the modern science and technology. However, in the same way as other composites the physical and mechanical properties are affected severely by different aging conditions like pressure, temperature, humidity and the curing condition. The absorption of water and plasticization of the composites deteriorates the service life, increases the chain mobility and decreases the glass transition temperature. It also reduces the mechanical properties of the composites. Thus, researchers are working on the effect of different aggressive environments on the durability of composites and to understand the changes in the physical and mechanical properties over the period of aging. This chapter deals with the different environmental aging conditions and their impact on the properties of the composites.