Invasive species are one of the major threats to biodiversity, ecosystem functioning, and socio-economic activities worldwide. Although scientific research has greatly advanced understanding of the ecological impacts of individual invasive species, ecosystems are increasingly experiencing multiple simultaneous invasions. Despite their growing prevalence, the ecological consequences of these co-occurring invasions remain poorly understood. In particular, the invasional meltdown hypothesis (IMH), which proposes that one invasive species can facilitate the establishment or impact of others, remains insufficiently explored at community and ecosystem levels. The X-IAS project aims to investigate the ecological dynamics and impacts of multiple invasive species within the same ecosystem, with particular emphasis on testing the IMH. As global environmental change and biological homogenisation continue to increase the frequency of co-invasions, understanding how invasive species interact is critical to accurately predict ecosystem responses and to develop effective management and conservation strategies. X-IAS focuses on the Minho estuary (Portugal), a well-known hotspot for invasive species that provides an ideal natural laboratory to investigate multi-species invasions. Three widespread and high-impact aquatic invaders have been selected as focal species: the Asian clam Corbicula fluminea, which can dominate benthic macroinvertebrate communities and modify sediment processes; the aquatic macrophyte Egeria densa, characterised by rapid growth and strong competitive ability; and the North American pumpkinseed fish Lepomis gibbosus, known to negatively affect native fish diversity and trophic interactions. These species are abundant in the study area and represent different trophic levels, enabling the investigation of complex ecological interactions within the estuarine food web. The main objective of X-IAS is to assess how interactions among multiple invasive species influence ecosystem structure and functioning. Specifically, the project aims to: (i) evaluate the ecological impacts of systems affected by multiple invasions by isolating and comparing the effects of the selected invasive species; (ii) experimentally test the IMH through controlled mesocosm experiments that simulate simplified versions of the estuarine ecosystem; and (iii) develop predictive models to describe interactions among multiple invasive species and forecast their combined impacts under different environmental scenarios, including climate-related changes such as increasing temperatures. The X-IAS project will generate new knowledge on the mechanisms and ecological consequences of co-invasions, addressing key gaps in invasion ecology. It will integrate field monitoring, mesocosm experiments, and modelling, aiming to provide evidence on how multiple invasive species interact and how they can shape ecosystem dynamics.