Helicopters are characterized by a high number of rotating parts. These include the main rotor, the tail rotor, the engines, the transmission shafts, etc. Other rotating and non-rotating parts such as pumps, bearings, dampers, and the landing gear system can also induce vibrations. These are not always easy to identify and correct. Historically there have even been cases of cells that after being put into service had to be discarded because of a tendency to give rise to high frequency vibrations that could not be corrected. Regardless of origins, a high level of vibration is a main obstacle to comfort and therefore also to the commercial placement of a helicopter. Since vibrations are in most cases related to the action of the rotors, the dynamic balancing of rotors and shaft is a very critical helicopter maintenance operation.