This paper presents an analytical approach to modeling guided Lamb waves interacting with linear and nonlinear structural damage. The active sensing process using piezoelectric wafer active sensors (PWAS) was modeled in the following four steps: (1) guided waves generation by transmitter PWAS (T-PWAS); (2) Lamb wave multi-mode dispersive propagation in the host structure; (3) linear and nonlinear interaction between Lamb waves and damage; (4) guided waves detection by receiver PWAS (R-PWAS). Structural damage was modeled as a new wave source, where guided waves are transmitted, reflected, and mode-converted. In addition, when guided waves interact with nonlinear damage, nonlinear higher harmonics will also be present. Real time sensing signal at R-PWAS was obtained, as well as the time-space wave field and the frequency-wavenumber representation. A Graphical User Interface (GUI) called WaveFormRevealer (WFR) was developed based on this analytical model. High frequency guided wave propagation in thick plates was done first. Beside fundamental modes (S0 and A0), higher wave modes were also observed. These analytical results were verified by experiments. Analytical simulation of linear interaction between Lamb waves and a notch was done next and compared with experiments. New wave packets due to mode conversion at the notch were observed. Subsequently, the nonlinear interaction between Lamb waves and a breathing crack was investigated using a contact finite element model (FEM). Distinctive nonlinear effects were noticed in both FEM simulation and analytical solutions. The paper finishes with summary, conclusions, and suggestions for future work.