ESDU 02020 is an overview of the various sources of noise from aeroplanes and helicopters and includes consideration of the propagation of that noise outside and inside the vehicle together with the various influences that may modify the noise before its perception by an observer. Various ESDU documents that provide methods for estimating noise are referenced at the point where those components are discussed. The propulsion system is first considered. In the case of gas turbine engines the contributions from the jet, fan, compressor, turbine and core are individually introduced. These contributions are further broken down, for the jet into jet mixing and shock noise and for the fan and compressor into blade passing frequency tones (including rotor-stator interaction effects), shaft-order tones and broad-band noise. Propeller and rotor noise are then discussed, followed by airframe noise including sonic boom. In most cases these various contributions to noise are illustrated to show their relative importance. Various methods of attenuating the noise are introduced briefly, including liners and barriers, and power-plant installation effects are considered. The effect of flight on noise measured under static conditions is explained and the various effects that the atmosphere has on attenuation are discussed together with the influence of ground reflection. Cabin noise due to airframe vibration, airborne noise and structure-borne noise is introduced. Finally, the various methods of modifying the measured noise to allow for its subjective perception by the human ear are detailed and explained.