In a broad context, the term photomovement encompasses any movement or its alteration induced by light. Photomovement is the result of the photoregulation of movement – which includes an entire complex of elementary processes caused by a light stimulus such as photoreception, primary reactions of the photoreceptor pigments, and the sensory transduction of the light stimulus into a physiological signal that governs the activity of the motor apparatus and results in the photoorientation of the organism.

The authors review the historical development and current state of the art in the biology of photomovement in algae and discuss problems in terminology and a logical basis for classification of photomovement in microorganisms. Their research has focused on two species of Dunaliella Teod., D. salina Teod. and D. viridis Teod., as the principal organisms investigated. The results of experimental investigations on the critical factors controlling and modulating photomovement are described and include the effects of various abiotic factors and critical aspects of photomovement.