Synchronization. The fourth important class of control goals corresponds to synchronization (more accurately, "controlled synchronization" as distinct from "autosynchronization" or "self-synchronization"). Generally speaking, synchronization is understood as concurrent change of the states of two or more systems or, perhaps, concurrent change of some quantities related to the systems, e.g., equalizing of oscillation frequencies. If the required relation is established only asymptotically, one speaks of "asymptotic synchronization". If synchronization does not exist in the system without control the problem may be posed as finding the control function which ensures synchronization in the closed-loop system, i.e., synchronization may be a control goal. Synchronization problem differs from the model reference control problem in that some phase shifts between the processes are allowed that are either constant or tend to constant values. Besides, in a number of synchronization problems the links between the systems to be synchronized are bidirectional. In such cases the limit mode (synchronous mode) in the overall system is not known in advance.