Organs motion in Radiotherapy induce an error in the dose received by the tumor. Therefore, this book has been published to evaluate the organs motion during external beam radiotherapy. It deals with the detection, measurement and analysis of the periodic physiological organs motion during external beam radiotherapy. During a fractionated course of radiotherapy, variations in patient position and in the alignment of beams will occur both intra- and inter-fractionally, and a margin for set-up error must be incorporated into the CTV-PTV margin. Errors maybe systematic or random. Systematic errors may result from incorrect data transfer from planning to dose delivery, or inaccurate placing of devices such as compensators, shields, etc. Such systematic errors can be corrected. On the other hand, random errors in set-up maybe operator dependent, or result from changes in patient anatomy from day to day, which are impossible to correct.