Microbes influence the ecology and evolution of their hosts in a variety of ways, including the formation of life-long beneficial or detrimental parasitic infections. Understanding the molecular and biochemical events that underpin symbiosis - beneficial or parasitic - has been a long-term goal of molecular symbiosis research. In addition to beneficial symbionts provisioning scarce resources to their hosts, a growing body of evidence shows that bacterial symbionts can protect their hosts from parasitic symbionts and predators. Here, we review recent theoretical predictions and experimental observations of symbiont-mediated protection in insects. We discuss the implications that protection has for the ecology and evolution of host, symbiont and pathogen and describe what is known about the molecular mechanisms that underpin symbiont protection.