The yellow mealworm, Tenebrio molitor , is currently one of the most important insect species produced for livestock feed and human consumption. High-density rearing conditions make the risk of disease and infections by parasitic symbionts a challenge in the mass production of these insects. However, certain symbionts are beneficial and should be favoured in order to promote healthy insect populations. Knowledge of parasitic symbionts and their management is essential for the insect rearing industry and its associated research. Here we review the documented microbial infectious agents, invertebrate parasites, and beneficial symbionts occurring in T. molitor . Furthermore, we discuss detection, prevention, and treatment methods for disease management in T. molitor production systems to inform future management and decision making in T. molitor rearing.