The field created by a bunch of charged particles as it passes through a beam pipe. The charges induced in the wall of a beam pipe by the leading edge of the bunch have an effect (usually bad) on the tail of the bunch. If the beam is centered, wakefields will cancel each other by symmetry, but if the beam is closer to one wall, the tail will be attracted to this wall resulting in a `banana’ shaped bunch (Transverse Wakefield). A wakefield also can be created in RF cavities by the loading effect: the tail gets less energy than the head of the bunch (Longitudinal Wakefield). The deceleration on parts of the bunch caused by wakefields can be up to 2 GeV.