<p>A popular hypothesis predicts that positive interactions involving non-native species will cause ecosystems to become more easily invaded and modified, resulting in an accumulation of invaders and their impacts—a process termed <i>invasional meltdown</i>. This model contrasts with the classical theory of biotic resistance, which emphasizes increasing antagonistic interactions among species as a community grows. Invasional meltdown can be an emergent consequence of serial or concomitant facilitations and their indirect effects. However, it has often been narrowly equated with simple biotic facilitation in which the establishment, population expansion, spread, or impact of one invader is enhanced through a positive interaction with another; the hypothesis has rarely been explored beyond these single pairwise facilitations. The original model emphasized continuous self-reinforcing effects of positive interactions of non-native species in general, but here we propose that invasional meltdown can arise through other mechanisms—including an influx of coevolved species, a cluster of interactions driven by a single keystone facilitator, and indirect interactions among both native and non-native species—all of which can result in an increasing invasion rate and cumulative, potentially synergistic impacts. Furthermore, invasional meltdowns are not likely to be indefinite; for example, they may be punctuated by periods in which the invaded community temporarily resists further invasion until a biotic or abiotic phenomenon disrupts the resistance and triggers another series of facilitations. Although invasional meltdown can potentially transform entire ecosystems, its ecological consequences at the community and ecosystem levels have rarely been tested, owing to the complexity of interactions and pathways involved.</p>