RF breakdown limits the operating gradient of a normal™]conducting
accelerating structure. This limit is important for future high™]energy
linear colliders because a higher gradient implies a shorter length and,
in general, a lower machine cost. RF breakdown is a complex problem in part
because several different physical processes are usually at work at the
same time on the surface of a typical accelerating structure operating at
a high gradient. In this talk talk we try to get at the underlying physics
for each of these processes and it will be discussed the activity status
on the X band accelerating structures.