Color vision is the ability to perceive differences in the wavelength content of a light source or an object. Color vision is, however, a complex process. For instance, depending on the spatial context, objects that omit or reflect the same wavelength content may have differently perceived color (e.g., in color induction). On the other hand, objects that omit or reflect different wavelengths may be perceived as having the same or very similar color (metameric colors or in color constancy). In any case, color vision starts with absorption of photons of different wavelengths and energies by the photopigments. In this chapter, the photopigments and the efficiency with which light of different wavelengths are absorbed by a photopigment are discussed. In addition, the translation of a photoisomerization to a photoreceptor excitation is considered as well as the signal transmission from the photoreceptors to post-receptoral cells and the post-receptoral processing of this signal in the retina. A large part of the chapter provides an overview of recent evidence that retinal processes in the major retino-geniculate pathways that are relevant for luminance and color vision, can be studied in the intact visual system by electroretinography (ERG), thus providing the possibility of directly studying the human retinal physiology. We also provide evidence that ERGs elicited by repetitive stimuli may be more efficient to study visual information processing in retino-geniculate pathways.