The three divisions of the ear (outer, middle and inner) each have an important role in hearing, while the inner ear is also crucial for the sense of balance. How these three major components arise and coalesce to form the peripheral elements of the senses of hearing and balance is now being studied using molecular-genetic approaches. This article summarizes data from studies of knockout and mutant animals in which one or more divisions of the ear are abnormal. The data confirm that development of all three divisions of the ear depends on the genes involved in hindbrain segmentation and segment identity. Genes that are regionally expressed in the inner ear can, when absent or mutated, yield selective ablation of specific inner-ear structures or cell types.