In recent years the authors have engaged in the development of a special technology to optimally modify a structure's physical parameters (mass, damping and stiffness) in time- dependent loading conditions. Specific efforts have been devoted to reduce structural responses under seismic loading. In this regard, there are many other research activities dealing with different devices and control algorithms generally classified in the following categories: passive control, active and hybrid control, adaptive control, semi-active control, etc. All together, this newly emerging field of structural control has generated significant momentum of development. Real-time structural parameter modification (RSPM) is a different approach from the categories mentioned above. A detailed explanation of RSPM is given in the references. In this paper, we discuss a number of fundamental issues facing the development of various structural control technologies. These issues are summarized from our development of RSPM, and are examined based on the view that structural control devices may be considered from only two distinct types: material type and mechanical type. Such a view helps to classify some advantages and disadvantages of different structural protective systems including RSPM.