The pineal gland is a tiny organ in the center of the brain that played an important role in Descartes’ philosophy. He regarded it as the principal seat of the soul and the place in which all our thoughts are formed. In this entry, we discuss Descartes’ views concerning the pineal gland. We also put them into a historical context by describing the main theories about the functions of the pineal gland that were proposed before and after his time.