The possibility of using photo-stimulus to control flow in microfluidics devices is very appealing as light can provide contactless stimulation, is biocompatible and can be applied in a non-invasive and highly precise manner. One of the most popular ways to achieve photo-control flow in microfluidic channels is throughout the use of photo-responsive molecules. We review here the different principles and strategies of using photo-responsive molecules to induce or control liquid motion using light, which include the use of photo-controlled polymeric actuators, photo-sensitive coatings, or photo-sensitive surfactants. We further analyse the capability of these approaches to induce flow control throughout the photo-operation of valves, photo-control of electro-osmotic flows or photo-manipulation of discrete microliter-sized droplets.