We present specialized ultrasound imaging modes for monitoring and guidance of noninvasive and minimally-invasive thermal therapy. One mode is based on two-dimensional imaging of temperature change using diagnostic ultrasound. We have validated this method both in vivo and in vitro in monitoring the heating patterns produced by noninvasive HIFU source and minimally-invasive RF ablation device, respectively. In addition, a nonlinear method for imaging the quadratic echo components from HIFU-induced lesions has also been developed and tested in vivo. Illustrative results from both modes of imaging are presented. These results demonstrate the unique advantages of ultrasound as an image-guidance modality. Specifically, the high spatial and temporal resolutions that allow for imaging highly-localized short-duration therapeutic and sub-therapeutic HIFU beams. With the advent of high-performance computing hardware, these imaging modes are now implementable in real-time. This will lead to active realtime monitoring and control of a range of thermal therapies in the very near future.