Unbalance is the most common rotor system malfunction. Its primary symptom is 1X vibration, which, when excessive, can lead to fatigue of machine components. In extreme cases, it can cause wear in bearings or internal rubs that can damage seals and degrade machine performance. Usually, whenever increased 1X vibration is detected, the immediate suspect is unbalance. However, because there are many other malfunctions that produce 1X vibration, many machines have been balanced only to have the real root cause problem reemerge. Thus, to properly diagnose unbalance, it is important to understand both the mechanism that produces unbalance and the other malfunctions that produce 1X vibration.