Imagine you are going to meet a friend and watch a movie. You are running late, the movie has started and your friend sends you a message to say they have gone in. You walk into the movie theatre and search for your friend’s face but have trouble finding them in the dark. Suddenly the screen lights up and you can see the seat numbers and faces of the people in the theatre. You manage to spot your friend, they see you and flash you a smile – you are forgiven your lateness! You begin to squeeze your way along the row to them. The movie features characters speaking in unfamiliar accents to you and at times you find yourself struggling to understand what is being said. But at least your seats are near the front so you can clearly see everything that is going on. This everyday situation contains multiple instances of a relation between physical stimuli and psychological responses. How do you tell the difference between your friend’s face and other faces? How do you identify their emotional state? How much light do you need to be able to see and then read the seat numbers? How do you judge whether you can squeeze along the row to them? How loud does the volume on the movie need to be for you to discriminate the different words being spoken? How close do you need to sit to perceive different levels of detail in the visual images? Psychophysics provides us with a theory and set of methods to answer these types of questions, essentially, methods that determine the relation between the intensity of a stimulus, the transformations it undergoes in the external and internal environment, and the sensation and subsequent response of an observer. This chapter provides an overview of the history of psychophysics, its classical methods and contemporary approaches to understand the relationship between the stimulus, noise and observer response.