The use of cell phone cameras has created a wash of image files that capture and preserve the world we see in two dimensions. Cell phones also provide a storage system that lets us carry large quantities of data around with us. This abundance of image data is more than just pictures; when combined with the ability to use and manipulate the image files, interesting science and other investigations become possible. Astronomers have known the value of digital image files since they first discovered that a film photogram could be digitized, i.e., converted to arrays of numbers. There are many types of image files commonly in use, and MATLAB® can make use of most popular commercial formats, or it can construct new ones. In science and engineering, quite often having an image alone is not sufficient; additional work must be performed to extract scientifically usable data. This requires image- and signal-processing skills to manipulate and filter the data. Image processing is a mathematical skill that is well supported by the capabilities of MATLAB. This is such an important area of work that specific collections of programs called Toolboxes are available from MathWorks to make image processing more accessible and usable. In this chapter we will learn how to read and write data image files in the more common formats and then introduce various means to analyze and process images using MATLAB. Spatial image filtering, which involves 2D convolution of a filter and an image, is explored in detail.