The number of insect species transported to non-native regions is increasing, and, once established, these invasive insects have serious impacts on the environment and regional economies. Recent research highlights several cases of insect invasions facilitated by symbiotic microbes. Symbioses impact biological invasions, but few reviews have addressed the role of symbiotic microbes in insect invasions. Focusing on the insect–microbial symbiosis, we show the importance of microbial symbionts in determining the pest status of insects at insect–microbial levels, insect–plant–microbial levels, and other multispecific levels. Drawing on examples from different ecosystems, we review the key mechanisms and principles whereby facultative/mutualistic microbes affect insect invasions and coevolve with the invasive insects. We propose a conceptual framework for assessing the role of symbiotic microbes in insect invasions that promises improved risk analyses, spread and impact modeling, and management of invasive insects.