Breast cancer is the most frequently diagnosed cancer (excluding skin cancer) and the no. 2 cancer killer in U.S. women. Due to recent advances in medical imaging, efficient screening and early detection of breast cancer have resulted to lower morbidity from the disease. Because of the successful detection of breast cancer at an early stage, treatment techniques have also improved. The premise of ablation techniques is that, if a tumor and its normal-tissue margin can be destroyed in situ, instead of being removed, the impact on the disease should be equivalent. In addition, if the mortality associated with operative intervention can be avoided, then the outcome using localized treatments may be more advantageous. Ablation techniques are, therefore, slowly emerging as less invasive, but equally effective, in the treatment of early-stage breast cancer, with high-intensity focused ultrasound (HIFU) being the only truly non-invasive, extracorporeal technique. In this chapter, the radiation force-based method of harmonic motion imaging (HMI) is described and proposed for monitoring of HIFU, an 1) entirely non-invasive (noncontact), 2) simple to implement, 3) real-time, 4) precise (estimating displacements of 1 mm to 10 mm), 5) fully integratable, and 6) low-cost technique for localized detection and in situ thermal treatment planning and monitoring of early-stage breast cancer. Most importantly, HMI's higher impact may lie in the fact that it can easily be applied in deeper-seated tumors with fewer alternative treatment options, such as in abdominal, e.g., liver and pancreatic cancers, preliminary feasibility of which will also be determined in vivo.