The Automatic Tube Current Modulation (ATCM) system is now available on individual Computed Tomography (CT) scanner manufacturers. The system modulates the tube current during scans according to patient size, shape and attenuation level. This is for optimizing tube current or patient dose while maintaining CT image noise level according to the user requirement. The principles of ATCM systems differ for different CT scanner manufacturers. The users should have knowledge and understanding of how modification to scan parameter will affect image quality and patient dose. Principles of ATCM system and factors affecting radiation dose for different CT ATCM systems have been summarized in this review article.