Thirst is fundamentally considered as a physiological function developed to maintain the homeostasis of body fluids. However, we occasionally experience thirst in situations that are not necessarily related to the maintenance of bodily fluid homeostasis. Because the only method available had been to quantify the degree of thirst using psychological indices, thirst research had made little progress until very recently. To quantitatively analyze thirst, it is necessary to elucidate the nature of the brain's "thirst center," which is believed to become active in response to thirst. Textbooks of physiology often refer to the "thirst center" in the hypothalamus, which is considered to sense the osmotic pressure of body fluids. However, they did not specify the location of this center in the hypothalamus. Furthermore, the existence of the so-called "osmotic pressure sensors" has yet to be confirmed. However, recently, a series of findings have been published that delve into the true nature of the thirst center. These advancements have been achieved using several new techniques, including the real-time monitoring of neural activities related to thirst regulation within the brains of experimental animals. At least at the animal level, recent advancements in experimental techniques have made it possible to objectively quantify the intensity of thirst as a physiological response. In this review article, the history of our research is presented and latest developments in thirst research are presented.