Hearing loss (higher-than-normal threshold for perceiving sound as a function of its frequency) is the final common pathway of various diseases affecting the ear and its nerve connection to the brain. Data on its prevalence and adverse consequences are used in the Health Insurance Study (HIS) to investigate the effect of different levels of insurance on health status and quality of care because of its common occurrence (particularly among older adults), ease of diagnosis, potential positive response to treatment, and impact on well-being. This volume reviews pertinent medical literature, details the HIS definition of hearing loss, describes HIS methods for measuring its presence and impact on people's lives, presents and discusses the enrollment data from the six HIS sites, and outlines quality-of-care criteria. Of 3,321 adult enrollees tested, 3.1 percent had hearing loss. Three-quarters of the hearing impaired reported worry about or activity restrictions from the problem.