This book presents essential topics in terahertz communications, including channel measurement, modeling, and security properties. Its chapters explore propagation mechanisms, multipath effects, and atmospheric impacts, and delve into advanced measurement techniques such as time-domain and frequency-domain methods. The book also provides insights into environment-specific channel modeling for indoor, outdoor, and aerial scenarios, as well as discussions of security challenges, encryption, and physical-layer safeguards. Real-world case studies highlight applications in 6G integration, wireless sensor networks, and the use of deep learning for performance evaluation. The book will appeal to researchers, engineers, and students interested in terahertz communication technologies.