Summary: 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. Here we formulate and discuss several methods for structural modification. The implementation of this technique is presented for real engineering structures, and the main problems faced by structural modification analysis are outlined.