Image processing refers to digital processing of any two-dimensional data (a picture) by a computer by applying signal processing methods. Image processing has a broad spectrum of applications, such as image representation, image enhancement or sharpening, image restoration by means of filtering, and geometrical correction. These applications are usually the first stage and input to the following stages in a computer vision system. In OpenCV, there is a specific module, imgproc, for image processing. In this chapter, we will cover the most important and frequently used methods available in the library, that is, pixel-level access, histogram manipulation, image equalization, brightness and contracts modeling, color spaces, filtering, and arithmetic and geometrical transforms.