<p>The vertebrate retina contains two types of photoreceptors – rods and cones – which are receptors for night and day vision respectively. These have a number of morphological and biochemical differences which determine their functional roles. The discovery of intermediate-type receptors in a number of vertebrates became the basis for the theory of transmutation, which postulates that both rods and cones are capable of changing their functional roles and transforming into the other type during adaptation of the visual system to different environmental conditions. Several levels of manifestation of changes in photoreceptor physiology during the functional transition can be distinguished: morphological (overall cell structure), molecular-biochemical (expression of specific isoforms of the light-sensitive signaling cascade proteins), and electrophysiological (sensitivity and kinetics of responses to light). Photoreceptors with a confirmed transitional type are found in the most primitive vertebrates, and also in groups which have undergone a shift in environmental conditions towards extremely low or high illumination. The last two decades have seen significant progress in understanding the molecular mechanisms of the functional transition from typical rods to cones and vice versa. However, a number of aspects remain poorly understood, primarily because many animals with transitional-type receptors are far from standard model objects of modern biology. This review examines examples of transitional photoreceptors in various taxa, describes the history of their study, and presents current research shedding light on the molecular features underlying their non-standard physiology.</p>