Signal demixing and parameter estimation for multiple overlapped signals in noise and interference background is a problem often encountered in radar， sonar， communication， navigation， and other fields. Improving parameter estimation performance under low signal-to-noise ratio conditions and robustness in the presence of model errors has always been the focus of research in the field of signal pro cessing and control. Aiming at the above problems， this book presents a general and robust relaxation based estimation method（RELAX） and introduces its basic principles and applications in many aspects.