## 1 Adaptive Structures Department, The Italian Aerospace Research Centre, 81043 Capua, Italy;  
[EMAIL_ADDRESS] (I.D.) Industrial Engineering Department, University of Naples \"Federico II\", 80125 Napoli, Italy  
Correspondence: [EMAIL_ADDRESS]  
Magneto-rheological fluids, or MRF, have been known for a long time in the technological and scientific community. They have great potential and, as well, have important showstoppers. This combination is always conflicting and may explain why applications of such excellent materials are not yet widespread, despite their market value having already topped the impressive figure of almost 1.5 billion EUR worldwide, and being projected to about 6 billion EUR in 2030. An MRF is a liquid that can change its rheological state when subjected to a magnetic field. Usually, viscosity is the main objective of such changes; its characteristic may span from values typical of water up to becoming almost solid. Since viscosity is targeted, the main uses concentrate on phenomena wherein this property plays an important role. Therefore, dynamic aspects are mainly considered, with the primary aim to variate the overall viscous damping function to increase the dissipation at the highest level. For instance, controlling viscosity may be essential for vehicle suspensions or other devices dealing with the impact of a system on the ground. Having almost no damping would cause the target structure to vibrate indefinitely, so a certain amount of dissipation capability is necessary. However, it cannot exceed a certain threshold, otherwise, the time response is dramatically affected, in turn determining the system to approach equilibrium in a very long time, virtually infinite. In other words, having infinite damping is almost equivalent, macroscopically, to have infinite local stiffness, as an additional constraint is added. Additionally, dissipation capability depends on velocity, so the damper should be tuned with its current value to attain the best performance. The potential of such an incredible instrument is hampered by its construction. Usually, a MRF is produced as an oil solution containing small metal particles. The magnetic-forced orientation of such particles hinders the fluid movement. Since metal is heavy, the particles tends to drop down, so that a continuous mixing shall be actuated even though the use of some additives can limit the issue. The drawbacks of this necessity are evident since it would require some external energy devoted to this operation which, in the case of a transport drum, is sometimes generated by a helix constantly minding the fluid. Luckily, some applications are self-consistent, as the structures that enjoy the presence of the MRF are themselves subjected to continuous oscillations. In the case of cars or bikes, this operation is strictly connected to continuous hits with road irregularities; for this reason, it is not surprising that many patents and several common implementations of magneto-rheological fluids are for road vehicles. Other major applications of MRF are related to seismic response attenuation, where the need to maximize the energy dissipation is crucial for guaranteeing the device's effectiveness and safeguarding the reference structure. In this case, the system may stand for a long time, silent, without anything to do. However, it should be ready to use at unpredictable moments. Therefore, its state should be preserved at each instant, and an automatic monitoring system is essential. This fact enlarges the costs of implementing such devices, even with the perspective of an exceptional response in danger. This Special Issue presents two papers that deal with the application of MRF on buildings. The first one [1] is an extensive review of many publications focusing on devicesfitted to certain structures. It deals with friction dampers (FD), tuned mass dampers (TMD), and viscous dampers (VD), further classifying them into passive, active, and semi-active systems.