Abstract. This review investigates the different pre-processing techniques like image registration and various methods under the category of image enhancement methodologies in digital mammogram. Furthermore, in this review a comparative analysis of the different pre-processing techniques are discussed along with the different techniques that are elaborately analyzed. This review is one step forward towards the identification of abnormality in digital mammography using CAD(computer aided diagnosis). Among the forthcoming techniques of pre-processing, followed by segmentation and feature extraction, before deriving the final conclusion whether a suspected region is malignant or not that still remains a major challenge.