abstract: (with Alessandro Rizzi) How can objects retain their color appearance under a great variety of lighting conditions? This is due to the ability of the human visual system to extract the invariant properties of colored surfaces under changing illumination conditions. The definition of a model of human color perception requires modeling this property, known as of color constancy. The Retinex theory, proposed by Edwin Land in 1971, has been one of the first attempts in this direction. Since then, many computational models based on Retinex have been proposed, often to serve as color enhancement techniques in digital imaging applications. In this paper, we present the experiments that led to the Retinex theory, describe its main postulates and discuss their implications in modern applications of color imaging.