The migration of solid fillers at the interface in emulsions was recognized for many decades and has been essentially used for morphology stabilization and for separation purposes in flotation or inverse-flotation processes. The interfacial location of solid fillers in molten polymer blends was also used for reinforcement purposes or for generating some physical properties such as conductivity. However, the true compatibilisation with the understanding of the interfacial activity criteria was not clearly identified until recently. The main objective of this presentation is to provide a comprehensive review on the compatibilization role of solid nanoparticles in immiscible polymer blends and blends undergoing phase segregation. While reporting the important facts on the current knowledge in the field, the aim here is not to report a jumbled regurgitation of literature data citing who has done what, but the focus is rather dedicated to the fundamental aspects of the mechanism of compatibilisation using a progressive approach based on proofs using both literature data and our own experiments.