  In this lecture the basic concepts of electromagnetic waves in accelerating
structures are discussed. After a short introduction on the propagation of
electromagnetic waves and on the concept of travelling wave and standing wave
structures, the most important parameters and figures of merit used to describe
the acceleration efficiency and the performance of accelerating structures in
terms of power consumption is presented. The process of RF design optimization
is also discussed. Finally a review of several type of structures using
different accelerating modes is given, with examples of interest for medical
accelerators.
