This account of the early history of the infrared spectral region is concerned with tracing the growth in understanding of the nature and properties of this radiation from the time of its discovery by Herschel in 1800 up to the first part of the present century. After workers in the first half of the 19th century had demonstrated the fundamental similarity of infrared and visible radiation, investigators during the second half discovered the materials and evolved the techniques of measurement and control of infrared radiation which provide the foundation for the current extensive use of infrared equipment in research, industry, and ordnance.