Inherited hearing impairment is a frequent and highly heterogeneous condition. Among the different subtypes of autosomal recessive non-syndromic hearing impairment, DFNB1 is remarkable for its high frequency in most populations. It is caused by mutations in the coding region or splice-sites of the GJB2 gene, or by mutations affecting regulatory sequences that are essential for the expression of this gene. GJB2 encodes connexin-26, a protein component of intercellular gap junctions, which play crucial physiological roles in the cochlea. Because of its high frequency, DFNB1 hearing impairment has received continued attention from researchers along the years, resulting in a wealth of data that is unparalleled among these disorders. Here we review our current knowledge on the genetic, molecular, and phenotypic aspects of this subtype of hearing impairment.