Epidemiological studies more than a century ago demonstrated that genetic factors are major contributors to hearing loss. Many genes associated with hearing loss have now been identified, although mutations in one of them, GJB2, explain a high proportion of genetic deafness in several populations. Environmental factors such as viruses (in particular, cytomegalovirus), ototoxic drugs, and noise also are associated with hearing loss, as is the aging process. Genetic hearing loss may be either syndromic (other organs and tissues are abnormal) or nonsyndromic. The majority show an autosomal recessive pattern of inheritance, but autosomal dominant, X-linked, and mitochondrial forms of hearing loss are found. In this article, four hearing loss syndromes, Jervell and Lange-Nielsen, Wolfram, branchio-otorenal, and Alport, are described; respectively, they are caused by mutations in genes encoding a potassium channel complex (KCNQ1, KCNE1), extracellular matrix proteins (COL4A3, COL4A4, COL4A5), transcription factors (EYA1, SIX1), and a protein that may be a novel endoplasmic reticulum calcium channel or a regulator of channel activity (WFS1). The contribution of mitochondrial DNA (mtDNA) mutations to hearing loss also is discussed.