This book highlights the principles and applications of terahertz time-domain spectroscopy (THz-TDS) in the fields of optics and electromagnetic waves. It offers a comprehensive description and in-depth exploration of the fundamental principles of THz-TDS systems, covering their hardware components and spectral data processing methods. The book also systematically introduces the applications of THz-TDS in various areas, such as crystal optics, food and agricultural products, biomedicine, cultural heritage conservation, and industrial nondestructive testing. Balancing comprehensiveness and accessibility, it serves as a valuable reference for researchers, educators, and professionals involved in THz-TDS research, education, and industrial applications.