This book presents High-Frequency Radars (HFR), extensively utilized for monitoring ocean surface currents and sea state along coastal regions. It serves as a comprehensive synthesis of essential principles underlying HFR, catering to readers without specialized expertise in any particular aspect. The first part of the book elucidates the principles and techniques necessary for interpreting radar signals in relation to ocean currents. Following this, the second part explores the physical mechanisms governing the propagation and scattering of electromagnetic waves across the ocean surface. Lastly, the third part offers practical examples and applications of HFR drawn from everyday practice. Tailored primarily for graduate students, and researchers in geophysics and oceanography, this book provides a comprehensive understanding of HFR technology and its utilization.