Hidden hearing loss (HHL) is a popular topic referring to complaints of hearing difficulty or perceived hearing loss despite having “normal” audiometric thresholds. Within the scientific literature, this term has most recently been used to refer specifically to the reduced amplitude of sound-evoked neural responses that occurs with loss of synapses that connect the inner hair cells (IHCs) to the auditory nerve. In other words, the patient’s audiometric difficulties are hidden behind a normal audiogram. However, perceived hearing loss despite normal audiometric thresholds may be a complaint related to numerous factors. The phenomenon of HHL including its epidemiology, pathophysiology, and clinical implications are under intense study and debate. In this review, we will address these subjects with primary emphasis in adult populations. Recent reviews of the work in animals and implications for patients are available, e.g., Liberman et al (2017). In addition, the review of this topic by Pienkowski (2017) is clinically oriented, and is highly recommended to the interested reader. Epidemiology of HHL Epidemiology refers to the study of patterns and causes of health and disease. To establish common epidemiological outcomes such as prevalence and determinants (factors that predict or are associated with outcome of interest), we must operationally define HHL. Here, we will define HHL as perceived hearing loss despite normal audiometric thresholds, i.e., normal hearing. Which raises the question, what is “normal hearing?” This sounds simple enough, but can be quite complex. The majority of epidemiological studies in the literature utilize the four-frequency pure tone average (PTA5124) of 500, 1000, 2000, and 4000 Hz and cutoff for normal PTA at greater than or equal to 20 or 25 dBHL. Currently, no large population-based study has examined extended high frequencies, i.e., greater than 8000 Hz, although several large data sets have become available. The reason that it is important to consider EHF frequencies is that depending on how you define “normal hearing,” you may over or under estimate the prevalence of HHL. Furthermore, even if an individual has “normal” hearing, that does not mean they have not acquired some threshold shift, as 10 years ago their hearing may have been as much as 15–20 dB better (lower thresholds) even if current thresholds are still within many common definitions of “normal” hearing. This content is an exclusive benefit for American Academy of Audiology members. If you're a member, log in and you'll get immediate access. Member Login If you're not yet a member, you'll be interested to know that joining not only gives you access to top-notch resources like this one, but also invitations to member-only events, inclusion in the member directory, participation in professional forums, and access to patient resources, tools, and continuing education. Join today!