​Genetic disorders are a common cause of congenital/early-onset sensorineural hearing loss. The identification of genes that are responsible for hearing loss is a breakthrough approach and has advanced our knowledge of the biology of hearing. Despite advances in the discovery of deafness genes, clinical application still presents problems because of the genetic heterogeneity of deafness. Genetic testing has become more important for highly accurate diagnosis, predicting the severity of hearing loss and associated abnormalities, selecting appropriate treatment options, preventing hearing loss, and better genetic counseling. In this book, the genes responsible for hearing loss will be reviewed, and a diagnostic strategy for mutation screening using a mutation/gene database based on the mutation spectrum of a corresponding population will be discussed. The simultaneous examination of the multiple deafness mutations by means of Invader assay followed by direct sequencing will enable us todetect deafness mutations in an efficient and practical manner for clinical use. We will discuss the genetic background of the pre-lingual as well as post-lingual deafness patients with cochlear implantation. Furthermore, auditory performance after cochlear implantation will be reviewed in the patients who underwent genetic evaluation. In addition to the above-mentioned factors, future screening for deafness genes will also be discussed in connection with the fact that many patients want to know their etiology, the most effective treatment, and helpful counseling about recurrence rates. Genetic counseling is important for the patients and their families to understand the heredity of hearing. Genetic counseling-based approaches are an important issue and when to perform genetic counseling will be discussed.   A new era is now approaching in which ENT clinicians will be able to access genetic testing.  This book, focusing on clinical application of genetic testing for deafness patients, will greatly benefit not only ENT clinicians including residents, but also geneticists and genetic counselors for clinical use.