In this Review of current clinical practice of set-up error verification by means of portal imaging, we firstly define the various types of setup errors using a consistent nomenclature. The different causes of set-up errors are then summarized. Next, the efforts done to minimize patient set-up errors are presented in different body sites. Also we focus on the more recent studies in order to assess the criteria for good clinical practice in patient positioning. We then present the result of our preliminary experience in the use of electronic portal image device (EPID) for the first time in our department. This a retrospective analysis for the images taken to patients treated with three dimensional conformal radiation therapy (3D-CRT) in our department between October 2005 and July 2006. Our results in shift of set-up errors reported in different tumor sites in the X, Y, Z directions in addition to errors in rotation. The number of images in our studies was still low compared to the literatures, but still there were images from different body sites. Finally we are giving some recommendations, collected from many publications in the use of portal imaging devices in the correction of systematic and random set-up errors during routine clinical practice.