The last two decades have seen considerable research activity on the use of riblets for viscous drag reduction. Experimental results concerning the performance of 3 M riblets on airfoils, wings and wing-body or aircraft configurations at different speed regimes are reviewed; these applications bring in additional effects like pressure gradients and three dimensionality. In addition to drag reduction, aspects of altered flow features due to riblets are discussed based on detailed wind tunnel measurements at low speeds. The available results obtained from wind tunnels as well as flight tests firmly establish the effectiveness of riblets from low speed to moderate supersonic Mach numbers. With optimized riblets, skin friction drag reduction in the range of 5-8% have been measured on 2D airfoils at low incidence and in mild adverse pressure gradients; strong evidence exist at low speeds to indicate that riblets are more effective in adverse pressure gradients. On wings of moderate sweep relevant to transport aircraft, riblets remain effective providing drag reduction comparable to 2D airfoils, as long as the local angle between the surface streamlines and riblet orientation is relatively small (o101). Limited data available on wing-body configurations show that total drag reduction of about 2-3% is likely. Certain suggestions for future research are outlined.