This book presents a guided wave-based structural health monitoring (GWSHM) system for aeronautical composite structures. Particular attention is paid to the development of a reliable and reproducible system with the capability to detect and localise barely visible impact damage (BVID) in carbon-fibre-reinforced polymer (CFRP) structures.TThe authors introduce a novel sensor installation method that offers ease of application and replacement as well as excellent durability. Electromechanical Impedance (EMI) is also explored to assess the durability of the sensor installation methods in simulated aircraft operational conditions including thermal cycles, fatigue loading, and hot-wet conditions.Damage characterisation using GWSHM is described and used to investigate damage in different CFRP structures. Key issues in guided wave-based damage identification are addressed, including wave mode and frequency selection, the influence of dynamic load, the validity of simulated damage, and the sensitivity of guided waves to impact damage in different CFRP materials.The influence of temperature on guided wave propagation in anisotropic CFRP structures is described, and a novel baseline reconstruction approach for temperature compensation is presented. Finally, a multi-level hierarchical approach for the quantification of an ultrasonic GWSHM system is put forth.