Our built infrastructure comprises an evolving organism, which requires monitoring and diagnostics throughout its service life in order to ensure a reliable and safe operation. To this end, Structural Identification (St-Id) exploits sensory feedback in order to reveal an abundance of information on the system throughout its life-cycle. The decoding of this information into an adequate representation of the system, may in turn serve as a tool for life-cycle assessment, maintenance planning and early prognosis and warning. This graduate level textbook and professional guide provides both an overview of well-established structural identification methods, as well as an introduction to selected advanced state-of-the-art tools, which are particularly suited for implementation in a practical setting. Both parametric and non-parametric methods are discussed, formulated in the time or frequency domain. The challenging issues of state and parameter identification are addressed. Moreover, special topics pertaining to nonlinear and non-stationarity behavior are further visited. The presented methods are demonstrated using MATLAB, with sample code provided for the featured examples.