Structural modification studies the relationship between the modal properties and spatial properties of a structure. Thanks to the rapid development of experimental modal analysis technology, structural modification has become a technique both for structure problem solving and for design optimization. Structural modification for the direct problem predicts the modal property changes from nominated spatial property changes and that for the inverse problem does the opposite. Several methods for structural modification are formulated and discussed. The implementation of this technique to real engineering structures is presented. In the summary, the main problems faced by structural modification analysis are outlined.