A video showing the flow of dye over and through a Nascar and Truck illustrating fluid mechanics and aerodynamics. In fluid dynamics it is critically important to see the patterns produced by flowing fluids, in order to understand them. We can appreciate this on several levels: Most fluids (air, water, etc.) are transparent, thus their flow patterns are invisible to us without some special methods to make them visible.On another level, we know the governing equations of fluid motion (the Navier-Stokes equations), but they are nonlinear partial differential equations with very few general solutions of practical utility. We can solve them numerically with modern computer methods, but these solutions may not correspond to nature unless verified by experimental results.On still another level the Navier-Stokes equations are pattern generators, and natural fluid flows display corresponding patterns that can recur on scales differing by many orders of magnitude. Such fluid patterns are familiar to almost everyone: the bathtub vortex and the tornado, the smoke ring and the mushroom cloud, the swinging of wires in the wind and the collapse of a historic bridge due forced oscillations from vortex shedding.Flow visualization is the art of making these patterns visible. In experimental fluid dynamics, flows are visualized by three methods: surface flow visualization, particle tracer methods, and optical methods. Surface flow visualization reveals the flow streamlines in the limit as a solid surface is approached. Colored oil applied to the surface of a wind tunnel model provides one example (the oil responds to the surface shear stress and forms a pattern). Particles, such as smoke, can be added to a flow to trace the fluid motion. We can illuminate the particles with a sheet of laser light in order to visualize a slice of a complicated fluid flow pattern. Assuming that the particles faithfully follow the streamlines of the flow, we can not only visualize the flow but also measure its velocity using a method known as particle image velocimetry. Finally, some flows reveal their patterns by way of changes in their optical refractive index. These are visualized by optical methods known as the shadowgraph, schlieren photography, and interferometry.In computational fluid dynamics the numerical solution of the governing equations can yield all the fluid properties in space and time. This overwhelming amount of information must be displayed in a meaningful form. Thus flow visualization is equally important in computational as in experimental fluid dynamics.