Geomembranes are popularly employed as part of composite liners in engineered landfills to control contaminant migration from the landfill. Volatile organic compounds (VOCs) present in the leachate can migrate through geomembranes by advection through defects or by diffusion. Among these processes, diffusion is an important contaminant transport phenomenon since diffusive transport may result in contaminant migration through intact geomembranes over relatively short times. In view of this, the objective of the present study is to provide an overview of the research carried out on diffusion of VOCs through geomembranes. The manuscript discusses the influence of diffusion on the contaminant transport process, for an otherwise welldesigned barrier system, on the long-term performance of the barrier system. The study also elaborates on the mechanism by which diffusion occurs through a geomembrane in response to a concentration gradient. Recent research on alternative methods to minimise VOC diffusion through geomembranes have been highlighted.