High precision radiotherapy has been a hot issue to radiation oncologists for the last 100 years. Introduction of mega volt machine with linear accelerators changed the paradigm of radiotherapy, and development of computers and three-dimensional treatment opened the new era of high tech radiotherapy. However, the change and development in high tech radiotherapy for the last 10 years was much bigger than those of previous century, and it seems that we are very close to the finish line of high precision radiotherapy. Linear accelerator has been a firm base for such an evolution to high tech radiotherapy. It is the machine which is used most frequently for stereotactic radiosurgery, image-guided and respiratory gated radiotherapy and this review will introduce the basic concepts of this linac-based high precision radiotherapy.