Membranes can be fabricated either from lipid or polymer molecules, leading to the formation of liposomes or polymersomes. In all types of liposomal membranes, the issue of phase separation plays a central role not only in the membrane-formation itself, but also in the resulting structural features taking place within or at the surface of such membranes. When nanoparticles or polymers interact with lipid membranes, the final morphology is strongly determined by the charge, composition and size of the interacting components, which in turn induce phase separation processes. The present review assembles investigations on lipid/polymer/nanoparticle interaction with the main focus directed towards model membrane systems published in the recent literature starting from 2005, providing a deeper and more thorough understanding of these complex interactions and their effects on membrane properties.