Global warming and its related issues have become a topic of concern in recent times. The most severe problem associated with increased environmental temperature is the higher heat load on the body. Passive heat exposure as such has many debilitating effects such as prickly heat rash, heat syncope, and other heat illnesses resulting in altered body functioning, mainly in elderly people and children. Working in a hot environment further increases heat load which can result in increased core body temperature (Tc) and can have severe outcomes. Thermoregulation is the homeostatic mechanism to maintain the Tc within a narrow range which is optimal for body functioning. Under a hot environment, sweat evaporation becomes the major thermoregulatory effector to reduce Tc. Loss of water and electrolytes in sweat, if not replenished, leads to dehydration which can be detrimental to human performance and survival. As temperatures are rising all around the globe, it is important to understand the challenges imposed on the body by dehydration and the importance of remaining hydrated. This book chapter has been conceptualized and designed to provide the necessary information about thermoregulation, core body temperature, heat illnesses, dehydration, and its consequences.