Abstract Reaction time is the time it takes for a person to respond to a stimulus. Reaction time has proven useful in assessing aspects such as processing speed or coordination and is widely used in disciplines such as cognitive psychology or neuroscience. This chapter gives an overview of the history of the method and covers its main contributions to different fields, with a special focus on CTIS. It will also address conceptual, ethical, and methodological issues, in an attempt to provide the reader with the necessary keys to understand its potential and applications in research.