Editor, Journals ASCE Dear Editor:Please find attached a Forum Paper we were invited, together with Dr. James Brownjohn, to write and submit to a special issue of the Journal of Structural Engineering dedicated to Structural identification. Together with Dr. Brownjohn we submitted a draft for review, revisions to which were advised by the Editors. These revisions were made and several additional comments were received. We are now submitting the paper after its second revision.Structural identification is a very important concept that will potentially help the civil engineering profession to understand the actual mechanical characteristics of constructed systems, incorporating the interactions between site, soil, foundations and the superstructure as well as their intrinsic and transient actions. Proper applications would lead to knowledge about the ground truth of as-constructed operating civil engineering products as opposed to empirical estimates of properties, performance and behavior during design. Given such a potential, structural identification is not a process but an art-form, requiring the leveraging of sufficient experience and expertise for modeling, field testing, interpreting the data and improvement of the models. Each of the writers has pioneered modeling, field testing and structural identification of a variety of constructed systems, each accumulating heuristics over 4 decades.In this paper the writers first articulated the state-of-the-practice of civil engineering and the pressing reasons for greater applications of structural identification to properly selected constructed systems. They continue with the history of structural identification as well as the challenges and opportunities facing this art in the 21st Century. As the writers became the carriers of the torch following an earlier generation of visionaries who laid the foundations of this art, they believe that this paper would serve to distill their decades of experience and heuristics for the future generations. We are slightly over length and hope that you will accept the manuscript based on its potential benefits for the civil engineering profession.