Entomopathogenic fungi (EPF) play a critical ecological role in regulating insect populations and maintaining the balance of terrestrial ecosystems. These fungi, especially species of Beauveria, Metarhizium, and Ophiocordyceps, serve as natural predators of many insects, helping to control their populations and prevent outbreaks. This chapter explores the multifaceted interactions between EPF and insects, highlighting the behavioral, physiological, and immune defences insects employ to resist fungal invasion, including grooming, thermoregulation, cuticular defences, and immune pathway activation. It also examines how plants modulate these interactions through chemical cues and volatiles that can either suppress or enhance fungal virulence, as well as through epicuticular waxes that affect spore adhesion. Some EPF can function as endophytes within plant tissues, providing dual benefits by promoting plant health and offering indirect protection against herbivores. Environmental conditions such as temperature, humidity, UV radiation, and soil composition further influence the efficacy and persistence of these fungi in natural habitats. Notably, some EPF exhibit host-specific behavioral manipulations, such as summit disease and death-grip phenomena, which enhance fungal dispersal and transmission. Collectively, these complex biotic and abiotic interactions determine the success of EPF in insect population regulation and their ecological significance in both natural and managed ecosystems. The chapter highlights the importance of understanding these dynamics for improving the use of EPF in integrated pest management (IPM) and predicting their responses under scenarios of environmental change. By bridging insights from entomology, plant biology, and microbial ecology, the chapter aims to provide a comprehensive understanding of the role and functions of entomopathogenic fungi in shaping insect population dynamics and contributing to ecosystem resilience.