The cochlea is divided into three compartments: the scala tympani, scala vestibuli, and scala media. The scala tympani and scala vestibuli contain perilymph, which is similar to extracellular fluid. The hair cells as the center for mechano-transduction are located in the scala media, which contains high K+ endolymph. The ionic equilibrium of cochlear fluids is essential to cochlear physiology as they allow the hair cells to receive mechanical stimuli. It is known that various genes are involved in fluid composition, and pathogenic variants in those genes disrupt cochlear homeostasis and impede normal hearing function. This chapter provides an overview of the potassium ion recycling-related genes (KCNQ4, GJB2, GJB6, KCNE1, KCNQ1) and the functional role of the anion exchange-related gene (SLC26A4). Among them, KCNQ4 (responsible for DFNA2) is known to be one of the most frequently observed responsible genes for autosomal dominant non-syndromic hearing loss (ADNSHL). A majority of the patients with KCNQ4 variants show high-frequency-involved hearing loss, although there are exceptions depending on the variants. GJB2 (responsible for DFNB1A, DFNA3) is known as the most common genetic cause of congenital hearing loss worldwide and therefore should be the first gene to be analyzed for congenital hearing loss patients. Hearing loss due to GJB2 variants can range from mild to severe, depending on the variants (there is a specific genotype/phenotype correlation), and is usually nonprogressive. Therefore, the degree of future hearing loss can be predicted, which is useful for selecting the most appropriate therapeutic intervention. SLC26A4 is known to be responsible for non-syndromic hearing loss associated with enlarged vestibular aqueduct (DFNB4) as well as Pendred syndrome. Hearing loss due to SLC26A4 variants is characterized by high-frequency-involved fluctuating and progressive hearing loss; therefore, intervention based on hearing loss at each stage is important. In addition to hearing loss, attention should also be paid to vertigo and goiter (thyroid function).