Athletes and regular sports practitioners are constantly attracted and motivated to seek new goals and personal achievements and push their body limits, even when it is hot! Although warmer and sunny days may, in principle, be an invitation to practice sports, we must always be aware of the adverse effects of heat on our body, especially when temperatures are higher. Heat stress will always be challenging when practicing sports and physical exercise. Hot ambiences may have an inherent risk of heat illness secondary to body hyperthermia, even in highly active individuals or athletes. This chapter, It’s Hot!, discusses physiological aspects of human temperature, the body’s thermoregulation, the harmful effect of heat combined with dehydration in sports performance, the importance of euhydration, and the diagnosis, predisposing factors, treatment, and prevention measures of clinical presentations of exertional heat-related illness.