Thermal strain imaging (TSI) is an ultrasound application which exploits the temperature dependence of ultrasonic echo time shift to form thermal strain images. The basic principle of TSI for medical examinations is that tissue expands and ultrasonic propagation speed changes when the temperature in tissue raises by using a directed-energy source to heat, causing relative shifts in scatterer position. The local temporal gradient of scatterer shift is often called thermal strain and thermal strain images are formed by measuring and restructuring thermal strain. In order to optimize the energy transfer and heating efficiency, a highly controllable energy source is a critical part of the TSI system. Ultrasound is the dominant energy source because it is noninvasive, nonradiative and can greatly simplify the probe design and its integration with existing scanners. As thermal strain is closely related to tissue components and temperature, the applications of TSI in biomedical field are mainly focused on tissue characterization and temperature monitoring. Although the application of TSI has been extensively studied, TSI currently can not be applied in clinical trials mainly due to the obstruction of tissue motion. To reduce negative effects of tissue motion, various methods for motion compensation have been attempted and good results have been achieved. We have reasons to believe that TSI will be generally welcomed as a new noninvasive clinical application with the improvement of energy source and the appearance of more effective motion compensation methods.