In　recent　years,　considerable　attention　has　been　paid　to　research　and　development　of　semiacitve　structural　control　systems,　and　great　progress　has　been　made.　However,　the　latest　research　results　show　that　there　still　exist　significant　limitations　for　engineering　application.　Some　key　problems　in　developing　the　semiacitve　control　system　are　presented　in　this　paper.　First,　a　detailed　review　of　semiactive　control　systems　is　provided　including　the　recent　advances　in　design　of　both　control　devices　and　control　algorithms.　The　semiactive　systems　which　are　reviewed　include　variable　stiffness　systems,　variable　fluid　dampers,　magnetorheological　dampers,　tuned　mass　dampers　and　tuned　liquid　dampers.　Next,　their　advantages　and　limitations　are　also　discussed.　In　addition,　some　suggestions　for　design　of　full-scale　control　devices　and　control　algorithms,　placement　optimization　of　control　devices　and　compensation　of　time　delay　are　also　given.　It　is　pointed　out　that:　(1)　the　traditional　variable　stiffness　system　can　only　adjust　the　frequency　of　the　structure　within　a　narrow　frequency　band,　and　may　be　negative　for　the　acceleration　response　control　of　the　structure;　(2)　only　far　from　the　resonance　may　the　variable　dampers　be　effective　for　seismic　response　control　of　structures　as　compared　to　the　passive　dampers.