Recognizing the elementary need for humidity control, the author presents a brief explanation of relative humidity, dewpoint, and air conditioning. The article outlines the development of hygrometers, from Leonardo da Vinci through the hair hygrometers of the 18th century, to the modern Salt-and-metal-wire and the wetand dry-bulb hygrometers. Refrigeration and desiccant dehumidification systems are discussed. Mention is made of ventilation and humidity control in underground facilities.