Understanding hearing means to understand that pressure variations in the air are transformed into a code that the brain will recognize. In order to reach that goal, the reader is first introduced in this chapter to a description of the physical characteristics of simple and complex sound waves. Fundamental notions of frequency, amplitude, and complexity are described and linked to the psychological notions of pitch, loudness, and timber. The basic biological structures responsible for hearing are also described, from the outer and middle ear up to the brain (auditory cortex). The description of the cochlea, the basic organ of audition, located in the inner ear is emphasized. Some classic theories of audition are reported for accounting in particular for the capability to decode a large range of frequencies. Finally, different causes of deafness and auditory problems are described.