Image enhancement on an image is a process to make it ready for further uses in certain applications. It enhances the visual property and the intelligibility of image thereby making the resultant image more beneficial for processing. This literature review covers a wide range of contrast enhancement techniques based on contrast manipulation for medical images such as CT, X-RAY, MRI and MAMOGRAM images. Also this review gives more detailed information about different novel medical image contrast enhancement techniques, efficient contrast enhancement algorithm approaches and so quality metrics which are useful and applicable in medical image analysis and clinical diagnostic synthesis.