ESDU 91010 describes the mechanism of galloping, that is an aeroelastic response in which changes in wind load due to structure response are in-phase with the structure velocity. The concepts of soft and hard oscillators are introduced and explained, and the possibilities of across-wind, along-wind and torsional galloping are discussed. Stall galloping, wake galloping and flutter as separate mechanisms are defined and briefly described. A method is provided for estimating the wind speed for the onset of galloping (or the structural damping required to prevent galloping at a given wind speed) for a single-degree-of-freedom system. A technique of approximately determining the galloping onset wind speed and the maximum amplitude is suggested for force coefficient curves typical of sharp-edged sections. Typical results for onset wind speed and response are shown for typical force curves. The theoretical approach used is fully explained and a worked example illustrates its use. Methods of reducing or eliminating galloping are outlined briefly.