  Knowledge of the electromagnetic interaction between a beam and the
surrounding vacuum chamber is necessary in order to optimize the accelerator
performance in terms of stored current. Many instability phenomena may occur in
the machine because of the fields produced by the beam and acting back on
itself as in a feedback device. Basically, these fields produce an extra
voltage and energy gain, affecting the longitudinal dynamics, and a transverse
momentum kick which deflects the beam. In this paper we describe the main
features of this interaction with typical machine components.
