The inner limiting membrane (ILM) represents the structural boundary between the vitreous and the retina, and is suggested to act as a barrier for a wide range of retinal therapies. While it is widely acknowledged that the morphology of the human ILM exhibits regional variations and undergoes age-related changes, insight into its structure in laboratory animals is very limited. Besides presenting a detailed overview of the morphology and composition of the human ILM, this review specifically reflects on the species-specific differences in ILM structure. With these differences in mind, we furthermore summarize the most relevant reports on the barrier role of the ILM with regard to viral vectors, nanoparticles, anti-VEGF medication and stem cells. Overall, this review aims to deliberate on the impact of species-specific ILM variations on drug delivery research as well as to pinpoint knowledge gaps which future basic research should resolve.