Computers play a fundamental part in all aspects of medical imaging. One of their most impactful roles is the ability to perform mathematical operations on digitally stored images in order to enable information of diagnostic importance to be visualised more readily. This chapter provides an overview of the most commonly employed image processing methods, including different types of filtering operations and both intensity and spatial transformations. These operations are described mathematically, and examples are provided to illustrate their effect on typical medical images. The impact of different processing methods on image characteristics such as spatial resolution and contrast is examined, and the potential for creating image artefacts (such as ringing) is discussed.