  Several indicators have pointed to the presence of an Electron Cloud (EC) in
some of the CERN accelerators, when operating with closely spaced bunched
beams. In particular, spurious signals on the pick ups used for beam detection,
pressure rise and beam instabilities were observed at the Proton Synchrotron
(PS) during the last stage of preparation of the beams for the Large Hadron
Collider (LHC), as well as at the Super Proton Synchrotron (SPS). Since the LHC
has started operation in 2009, typical electron cloud phenomena have appeared
also in this machine, when running with trains of closely packed bunches (i.e.
with spacings below 150ns). Beside the above mentioned indicators, other
typical signatures were seen in this machine (due to its operation mode and/or
more refined detection possibilities), like heat load in the cold dipoles,
bunch dependent emittance growth and degraded lifetime in store and
bunch-by-bunch stable phase shift to compensate for the energy loss due to the
electron cloud. An overview of the electron cloud status in the different CERN
machines (PS, SPS, LHC) will be presented in this paper, with a special
emphasis on the dangers for future operation with more intense beams and the
necessary countermeasures to mitigate or suppress the effect.
