As marine organisms respond to a warming ocean, we have observed a rapid redistribution of marine biodiversity, globally. The most common consequence has been a trend toward increasing representation within communities of species with affinities to waters warmer than the historical norm for the given location, a phenomenon now known as “tropicalisation”. Driven by differing dynamics at leading and trailing range edges of the species comprising the local community, tropicalisation leads to the progressive homogenisation of marine communities. These interactions between changing environmental conditions and the development of new, more homogenous communities challenges our understanding of how marine ecosystems function. Although much is left to learn, the rapid accumulation of evidence from observational, experimental, and modelling studies provides insights into what topicalisation might mean for marine communities and the people who depend on them as we progress through the 21st century.