The present state of knowledge on scale effects at high lift and low speeds, at transonic speeds, and on aircraft drag are presented. In addition, scale effects in various important specific scale-sensitive areas such as forebody vortex flows, the flow in and near open cavities, the flow into an air intake, the flow over propellers, and on ice accretion simulation testing are discussed. The emphasis is on scale effects that have been observed in flight-tunnel comparisons for specific aircraft. It is concluded that much has been learned about scale effects: however, precise prediction can still be difficult. Twenty recommendations for further research are mentioned. This AGARDograph has been produced at the request of the Fluid Dynamics Panel of AGARD.