The visualization of the velocity field in fluid flows is discussed on the basis of the nearly disturbance-free method of photochromic coloring. Several examples are given which show that other methods of visualization would fail to get insight into the flow behavior. For nonpolar organic liquids the photochromic material 1,3,3-trimethyl-6’-nitroindoline-2-spiro-2’-benzpyran was used. Stationary and non stationary motions of liquids are investigated, and the application to two-phase flows is pointed out. Especially the use of UV-laser light is shown to be profitable for generating colored patterns in the flow field which allow the determination of the velocity profile and the velocity gradient in the flow field. Recent research in the field of water soluble photochromic material has led to the application of the laser induced photochromic coloring even in large scale experiments.