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Ewald Hering (August 5, 1834 - January 26, 1918) was a German physiologist who did much research into color vision and spatial perception. Hering disagreed with the leading theory developed mostly by Thomas Young and Hermann von Helmholtz. Helmholtz's theory stated that the human eye perceived all colors in terms of three primary colors: red, green, and blue. Hering instead believed that the visual system worked based on a system of color opponency, a proposal now widely recognized as correct.

Hering looked more at qualitative aspects of color and said there were six primary colors, coupled in three pairs: red-green, yellow-blue and white-black. Any receptor that was turned off by one of these colors, was excited by its coupled color. This results in six different receptors. It also explained afterimages. His theory was rehabilitated in the 1970s when Edwin Land developed the Retinex theory that stated that whereas Helmholtz's colors hold for the eye, in the brain the three colors are translated into six.

Heinrich Ewald Hering attended the universities of Prague and Kiel, receiving his doctorate in 1893 at Kiel, where he worked in the institute for general and experimental pathology 1893-1898. He was habilitated for general and experimental pathology in 1895, becoming professor extraordinary in 1901, professor in 1903. In 1913 he followed a call to Cologne, as professor of physiology.

Hering was not a very popular man, but he was an important medical scientist, recognized for his thoroughness and stubbornness. His research concerns the normal and pathological physiology of the nervous system, muscles, heart, blood vessels and the autonomic nervous system. His work on the automatic regulation of the circulation by the pressoreceptor nerves earned him a recommendation for the Nobel Prize in physiology or medicine, but he did not receive it.

In 1861 he described an optical illusion which now bears his name - Hering illusion.

He also developed Hering's law of equal innervation to describe the conjugacy of saccades in animals.

• R. Steven Turner, In the eye's mind : vision and the Helmholtz-Hering controversy (1994, Princeton University Press).