In this chapter we will briefly report only the main aspects of this vast phenomenology, focusing only on those related to the topic of this book, i.e., the OAE-based hearing diagnostic techniques and the importance of establishing a coherent theoretical and phenomenological framework for their effective use. A main distinction may be made between psychoacoustics, that studies the perception of sound by analyzing the listener’s subjective response, and physiological acoustics, that performs objective measurements of either electrical or acoustical quantities, related to the response one or more physiological elements of the hearing system. A large part of psychoacoustics will be neglected, mentioning only the basilar concepts that can be useful to assess the effectiveness of diagnostic techniques based on OAEs by comparison with their psychoacoustic “counterparts”.