Water is the most abundant molecule in the human body, and thirst is the key survival instinct in body fluid homeostasis. Osmolality and plasma volume are the main thirst-regulating factors, which also stimulate kidney water conservation via arginine vasopressin hormone secretion from the posterior pituitary. Other factors such as mouth dryness, food consumption (especially dry), and even efforts to relieve discomfort due to spicy food consumption also stimulate thirst. Interestingly, following dehydration-induced thirst, it appears that humans stop drinking after having free access to water, well before body fluid restoration is achieved, probably because of oropharyngeal receptor activation. For instance, in sports, measures of urine concentration show that athletes tend to start training and/or competition in a suboptimal hydration state. Moreover, during exercise, athletes rarely drink more than two-thirds of their sweat losses, thus experiencing involuntary hypohydration. All of the above findings suggest that thirst (a) gets activated when significant hypohydration occurs and (b) may not be the most effective signal for maintaining optimal hydration.