Mechanical devices easily cause vibration because they are constructed with structural materials that have little internal damping. For this reason, vibration control has long been a big problem for the development of excellent machines. Now, sophisticated vibration control technology is becoming indispensable for satisfying various demands, related to the higher performance, reduced weight, energy savings, etc. of machines, which have become increasingly stronger in recent years. In particular, a large number of problems in which active vibration control holds the key are occurring in the most advanced fields of engineering. As can be seen in various examples of super-tall buildings such as the Yokohama Landmark Tower and Tokyo Gas Building, which have recently been completed at various locations, the construction of new structures like super-tall buildings has become possible by the support of this technology. On the other hand, with the further progress in mechatronics, it is now common sense that a control system is incorporated in any of today's machines. However, this has caused a new problem related to vibration. The problem is that energy injected for controlling position or motion excites vibration characteristics neglected from the control object and induces violent vibration in the machine. To be more specific, a flexible rotor controlled by a magnetic bearing is; capable of rotating at ultra-high speeds, but its flexible vibration must be controlled in order to solve a multiple 'number of critical speed passage problems. At such a time, higher-order vibration modes neglected from the object of control may cause unstable vibration. This is a new problem called spillover instability. It is expected in the future that an increasing number of such problems related to the simultaneous control of motion and vibration will arise in mechatronics equipment. Up to now, for the control of vibration, passive vibration controlling devices which do not require the injection of energy from outside have often been used. However, with the recent demand for sophisticated vibration control technology as described above, active vibration control methods using sensors, actuators, and controllers have suddenly attracted attention. In the background of the realization of such vibration control methods is the fact that modern control theory, which was considered at the outset to be difficult to handle and hard to put immediately into practical use, and the subsequently developed robust control theory have become easily usable as supported by the following developments: * Development of control system design supporting software as represented by MATLAB and SIMULINK. * Progress in hardware with improvements in computers' computational speeds and with the appearance of DSPs. * Progress in electromagnetic force utilization technology made possible by the development of new materials such as high-performance magnets. * Advancement of vibration visualization technology for control objects based on the development of theoretical and experimental vibration analysis methods. * Advances in accurate control modelling methods. In particular, although these control theories are difficult to make use of unless accurate models of control objects are created, this difficulty has been solved due to the advances in the methods for the optimum placement of sensors and actuators based on experimental modal analysis and also because of the progress in the modeling methods. In this way, these theories are now about to contribute substantially to the development of vibration control technology, and there is even a view that vibration control is being act ively utilized as a splendid place for the testing of new control theories. Thus, in vibration control, vibration analysis and control theory are beginning to develop in a balanced operation like two wheels of a vehicle. Against this background, it has been decided to feature active vibration control in this issue and the next. This issue consists of two explanatory articles on examples of active vibration control and magnetic bearing control problems, eight articles mainly dealing with the active vibration control problems related to flexible structures, two technical reports on the vibration control of super-tall buildings and main towers of large bridges, and an introduction to the research laboratory in Japan where the concept of the vibration control of super-tall buildings was first proposed and realized. At the time when weight reduction is being sought from every field, the slimming and flexibility of structures as well as their resulting vibration control problems cannot be avoided. From this point of view, this special issue has been compiled centering on articles dealing with vibration control problems for flexible structures and their concrete structures. This issue was edited by Seto of Nihon University. Kazuo Yoshida of Keio University will be in charge of the next issue. The editor is most pleased if this special issue draws attention of its readers.