This book introduces proposed new standards between digital images, which recorded the best results for the similarity between two images, although the images were exposed to a large amount of noise, and a peak signal to noise scale )PSNR)was used. The reference to noise as a measure of the noise ratio in the image processing and computer vision measuring image similarity among the two images becomes a fundamental case in many problems. In recent years, many images of similarity measures were proposed, most of them work with very particular kind of image distortions. Some of them are viable to a wider range of applications such as the structural similarity measure (SSIM) index. This chapter presents an introduction to image similarity, similarity methods and explains the most related works in detail; also it presents a literature survey for image similarity measures.