Objective To review basic and clinical findings relevant to defining temporary (TTS) and permanent (PTS) threshold shifts and their sequelae. Data Sources Relevant scientific literature and government definitions were broadly reviewed. Data Synthesis The definitions and characteristics of TTS and PTS were assessed and recent advances that expand our knowledge of the extent, nature and consequences of noise-induced hearing loss were reviewed. Conclusions Exposure to intense sound can produce TTS, acute changes in hearing sensitivity that recover over time, or PTS, a loss that does not recover to pre-exposure levels. In general, a threshold shift ≥10 dB at 2, 3 and 4 kHz is required for reporting purposes in human studies. The high-frequency regions of the cochlea are most sensitive to noise damage. Resonance of the ear canal also results in a frequency region of high noise sensitivity at 4–6 kHz. A primary noise target is the cochlear hair cell. While the mechanisms that underlie such hair cell damage remain unclear, there is evidence to support a role for reactive oxygen species, stress pathway signaling and apoptosis. Another target is the synapse between the hair cell and the primary afferent neurons. Large numbers of these synapses and their neurons can be lost after noise, even though hearing thresholds may return to normal. This affects auditory processing and detection of signals in noise. The consequences of TTS and PTS include significant deficits in communication that can impact performance of military duties or obtaining/retaining civilian employment. Tinnitus and exacerbation of post-traumatic stress disorder are also potential sequelae.