Some　basic　concepts　about　the　active　structures　were　firstly　explained,　and　the　main　subjects　to　study　in　the　field　of　active　structure　dynamics　were　synthesized.　For　the　linear　active　structures,　the　annotations　on　the　modes　were　done　in　detail.　The　physical　meanings　of　the　right　and　left　eigenvectors　were　explained.　The　right　eigenvectors　are　the　modal　shapes　and　the　modal　responses　of　an　active　structure　depend　on　the　left　ones.　The　adjoint　structure　of　an　active　structure　was　defined　and　the　reciprocity　theorem　was　interpreted.　For　two　active　structures,　which　are　adjoint　to　each　other　and　with　the　reciprocal　gain-matrices,　the　right　and　left　eigenvector　are　reciprocal.　The　relationship　between　an　active　structure　and　the　corresponding　passive　structure　is　expressed　with　the　transfer　functions,　which　is　employed　to　resolve　the　estimation　problems.