Publisher Summary This chapter defines how to use the MATLAB® software to do psychophysics. Psychophysics deals with the nature of the quantitative relationship between physical and mental qualities. The practice of psychophysics is ubiquitous in all fields of neuroscience that involve the study of behaving organisms, be they man or beast. Curiously enough, the origins of systematic psychophysics can be traced to a single individual. The key to visual psychophysics is to elicit relatively simple mental phenomena that lend themselves to quantification by presenting physical stimuli that are easily described by just a few parameters such as luminance, contrast, or spatial frequency. One way to create these stimuli is to use commercially available graphics editors, most prominently Photoshop®. While this practice is very common, it comes at a cost. The chapter also introduces methods by which one can create and present visual stimuli of any kind on the screen. MATLAB includes a large library of adequate functions and it visualizes images by assuming triples in a 256-element RGB space. Each element of the triplet has to be an integral value between 0 and 255. This corresponds to a range of 8 bit. Hence, these elements can be represented by variables of the type uint8.