Karst voids at pile locations can have a significant negative impact on the bearing capacity and overall safety of cast-in-place pile foundations. This book introduces a state-of-the-art detection system, comprising of specialized equipment, algorithms, and software, designed to identify karst voids during deep cast-in-place pile foundation construction. The system includes a unique multifrequency borehole sonic probe, a single borehole radar, two pile hole sonar probes, and a corresponding signal analysis method. Several field and synthetic tests have been conducted to detect karst voids during cast-in-place pile foundation construction, and the results demonstrate that this technology can offer detailed geological information for pile design and ensure the safety of foundation construction. The work presented in this book has received the First Prize for Technological Invention in Shanghai, China in 2022.