This chapter covers definitions and classifications of hearing loss in order to providea foundation on which the main structure of the book is built.After an introduction on some of the difficulties with terminology in this field, thenext section of the chapter defines the different types of hearing loss, and attempts tobring clarity to the way that different terms are used to describe hearing difficulties. Itincludes causes of hearing loss along the auditory pathway as well as in the ear.The third section provides a classification of hearing loss in terms of (1) its types,location and timing; (2) quantification of hearing loss, according to the different tests ofhearing loss that are currently in use; (3) international systems of classification developedby the World Health Organization (WHO), and information on systems used or proposedby other bodies.© 2012 Nova Science Publishers, Inc. All rights reserved.