Frequency Response Analysis (FRA) is a powerful diagnostic tool that offers deep insights into transformer health, though its interpretation presents significant challenges. This book introduces advanced techniques and intelligent methodologies for assessing transformer winding distortion and deformation through FRA. Drawing on extensive industry experience and academic research, it explores various methods for evaluating transformer winding conditions and presents in-depth analyses of fault mechanisms and failure modes from real-world data on over 80 power transformers. The causes, symptoms, and consequences of component failures are identified, offering valuable insights into transformer health. An advanced transformer modeling method is introduced to improve fault simulations, which can also be used for overvoltage prediction and transient analysis. Real-world case studies offer practical guidelines for interpreting FRA measurements, contributing to more accurate diagnostics and informed decision-making regarding transformer maintenance and replacement. This comprehensive resource serves as a valuable reference for professionals and researchers aiming to improve transformer reliability and optimize power system performance.