Aerodynamics and hydrodynamics are still the main domains that make greater use of flow visualization and classical optical techniques such as schlieren and interferometry than of more recent techniques such as holography speckle, laser light sheets, laser-induced tracers and laser-induced fluorescence. A number of studies are now under way on turbulent and vortex flows, within boundary layers or wakes, in the mixing layer of two flows. Other studies concern jets, two-phase flows and air-water interface. To review and discuss developments in flow visualization, four international symposia have been held. Following Tokyo, Bochum and Ann Arbor, the Fourth International Symposium on Flow Visualization (ISFV 4) was held in Paris in August 1986.