ESDU 91012 is particularly applicable to lightly damped structures typical of those used in aerospace. The principal sources of damping (acoustic, material and structural) are discussed and typical values of damping ratio likely to be met in various types of metallic and composite structures without special treatment are suggested. The properties of materials used to provide additional damping are reviewed, with particular consideration given to temperature and frequency effects. Factors to be considered in practice when excessive noise or vibration are to be alleviated are discussed. Methods of increasing damping in structural design (metallic or composite) are considered and the addition of damping by layered treatments or a tuned damper is discussed. Analytical methods for estimating the damping provided by added free layers of damping material applied symmetrically or asymmetrically to beams and plates are given. The damping due to constrained single or multiple layers of damping material also applied to beams and plates is discussed and behaviour of loss factor with variation in layer thickness is illustrated. The effect of multiple layers and use of different materials in combination is also considered. Good practice in the use of the layered treatments and tuned dampers is discussed and illustrated in particular cases.