Images are an important part of today’s life. Images are captured through many devices such as cameras, mobile phones, scanners etc. The quality of images is degraded due to many reasons. The reasons may include adverse weather conditions, motion of objects or less power of capturing device. To improve these images for human viewing or to make it suitable for further analysis, it is necessary to enhance the quality of images. Image contrast enhancement is one of the methods to improve image quality. There exist a number of contrast enhancement techniques. This paper provides overview of basic concepts and various techniques of contrast enhancement. This paper mainly focuses on histogram modification based contrast enhancement techniques.