The article reviews the mechanisms underlying the impacts of exotic plant invasions on terrestrial ecosystems, focusing on community structure and ecosystem processes. Despite the well-recognized impacts of exotic plants, the pathways through which these impacts occur are poorly understood. The authors analyze over 150 studies to evaluate the mechanisms behind the effects of exotic plants on plant and animal community structure, nutrient cycling, hydrology, and fire regimes. They find that while many studies examine the impacts on plant diversity and composition, few test whether these effects arise through competition, allelopathy, or other processes. Competition is often hypothesized as a mechanism, and studies comparing native and alien plants generally find strong competitive effects of exotic species. However, the most severe impacts are often mediated by changes in fire regimes, which can alter community structure and ecosystem processes. The review highlights the need for more mechanistic studies to understand how invader traits compare with those of the resident community, and to explore how communities resist invasion. The authors also discuss the relationship between the controls over invasibility and the controls over impact, suggesting that understanding these relationships is crucial for predicting which species will exert the greatest impacts.The article reviews the mechanisms underlying the impacts of exotic plant invasions on terrestrial ecosystems, focusing on community structure and ecosystem processes. Despite the well-recognized impacts of exotic plants, the pathways through which these impacts occur are poorly understood. The authors analyze over 150 studies to evaluate the mechanisms behind the effects of exotic plants on plant and animal community structure, nutrient cycling, hydrology, and fire regimes. They find that while many studies examine the impacts on plant diversity and composition, few test whether these effects arise through competition, allelopathy, or other processes. Competition is often hypothesized as a mechanism, and studies comparing native and alien plants generally find strong competitive effects of exotic species. However, the most severe impacts are often mediated by changes in fire regimes, which can alter community structure and ecosystem processes. The review highlights the need for more mechanistic studies to understand how invader traits compare with those of the resident community, and to explore how communities resist invasion. The authors also discuss the relationship between the controls over invasibility and the controls over impact, suggesting that understanding these relationships is crucial for predicting which species will exert the greatest impacts.