Beam coupling impedance (in other words, wakefield) causes collective beam instabilities, heating and degradation of components due to the electromagnetic field of the beam, and beam-induced electromagnetic interference. These effects are outlined from an experimental point of view. Examples of beam ducts and kickers are mainly from J-PARC, but examples from other institutions will also be presented. Countermeasures such as kicker modifications and transverse feedback dampers will also be presented.

The LHC is approaching the end of its third operational run, with machine protection and performance having demanded an excellent control of the single-particle dynamics. Additionally, the requirement to rapidly commission multiple diverse sets of optics configurations within each year, and from year-to-year, placed clear demands on the measurement and correction methods employed. Tight tolerances on the linear optics have been consistently achieved, with the drive to ever-more pushed optics for the High Luminosity LHC era continuing to introduce new challenges. Routine control of linear coupling has been an operational necessity, while significant progress has also been made extending the understanding and control of the optics into the nonlinear regime. This paper presents the key methods used, the results obtained, and discusses the challenges to control of the beam-optics in the LHC.