In modern surgery, intraoperative assessment of blood perfusion is an urgent but still unresolved problem. Due to the fact that light has minimal impact on biological tissue, optical methods are the most promising for non-contact measurements of blood flow in vivo. Nevertheless, despite many years of extensive studies of blood flow parameters by means of optics, the fundamental question of the interaction of light with living biological tissue containing blood vessels remains insufficiently studied. For example, the reasons why light, after its interaction with tissue, is modulated at the heart rate are still a matter of debate. This review will present a number of experimental observations that cannot be explained within the framework of the generally accepted models of the light interaction with living tissue. The question of how the observed modulation of light is related to the parameters of blood flow and its changes during pharmacological or surgical interventions will be discussed. Special attention will be paid to the discussion of systems of imaging photoplethysmography synchronized with an electrocardiography and laser speckle contrast imaging, which allow the assessment of tissue perfusion in a wide angle of view and with high resolution in both space and time. Since imaging photoplethysmography and laser speckle contrast imaging rely on fundamentally different principles of the interaction of light with biological tissue in vivo, their complementary (multimodal) use could provide an in-depth comprehensive assessment of hemodynamics in clinical setting. Keywords: light interaction with tissue, intraoperative monitoring, blood supply, imaging photoplethysmography, laser speckle contrast imaging.