This book provides an in-depth and systematic introduction to transmissibility-based system identification and structural health monitoring (SHM). It establishes transmissibility as a paradigm-shifting framework for SHM, advancing beyond traditional input-output and output-only methods to propose a unified output-output paradigm. This innovative approach eliminates reliance on excitation measurements or restrictive assumptions such as white noise inputs. Drawing on over 15 years of original research, spanning more than 30 high-impact publications by the author, the work delivers new contributions ranging from theoretical advancements in transmissibility's physical/statistical properties and novel identification algorithms to practical SHM applications validated through numerical simulations, laboratory experiments, and field implementations. The detailed exposition of formulations and implementations empowers engineers to directly apply these concepts in practice, while researchers in academia and industry will find inspiration to pursue further investigations in SHM.