There is growing evidence of the importance of microbial mutualistic symbioses in insect-plant interactions. Mutualists may affect host plant range and enable insects to manipulate plant physiology for their own benefit. The plant can also be a route for the horizontal transfer of mutualistic microorganisms among their host insects. Where this occurs, selection for improved transmission might cause the insect mutualist to damage the plant and become a plant pathogen. Insect microbial associates can influence ecological communities by changing the way the plant interacts with their hosts' competitors and natural enemies. We review recent research in this field and suggest that insect mutualists may be more important 'hidden players' in insect-plant interactions than is currently realised.