This book provides a wide view of time delay estimation technique, which is common technique in Digital signal processing and involved in several applications that including Radar, Sonar, earthquake detection, weather foresting & seismic exploration.These applications are particularly used to find out the time-differences-of-arrival between signals received at an array of sensors. In this manuscript, a step by step review on a variety of mathematical approaches used to estimate the time delay based on correlation technique. A comparative study is reported between them with aim of examining their performances. Algorithms for accurately computing the time difference between two sensor outputs using real seismic data sets. A low cost earth motions detection system with sensitivity less than 5 Hz. Finally, this book is considered a good hand out to guide people who manipulate digital data in different disciplines.