The senses of hearing and balance are important for performing daily tasks. This review discusses the biology and anatomy of hearing and balance, as well as the development and use of prosthetic devices for augmenting and/or replacing these important senses. The authors describe the history of the human auditory system going back to the 15th century. They also discuss the biology of the vestibular system, which is involved in balance, perceiving motion and spacial orientation. Complex interactions in this system make it challenging to design a prosthetic device for replacing a damaged vestibular system. The authors conclude that improved understanding of the structure and function of the vestibular sensory receptors (i.e., cristae and maculae), as well as the auditory sensory receptor (organ of Corti), has been crucial for developing and applying cochlear implants for treating deaf individuals and those with poor hearing. Growing knowledge should be useful for designing and constructing a vestibular prosthesis for treating individuals with life-altering balance disorders in the future. The goal of the article is to establish a theory on molecular evolutionary changes involved in cell types and anatomy of the ear present in vertebrates alive today. The authors discuss the evolution of the ear and various hypotheses involved. The acousticolateralis hypothesis states that the inner ear originated from the lateral line, but there is no convincing evidence showing that this hypothesis is correct. The statocyst hypothesis states that the small statocysts, which contain ciliated sensory cells of crustaceans, coelenterates, ctenophores, and echinoderms, are precursors to the vertebrate ear. This suggests that vertebrate ears and hair cells have a common origin and evolved together. A more recent hypothesis called the “mechanosensory cell first” hypothesis states that the mechanoreceptor, and therefore its molecular machinery required to form the mechanoreceptor, is the link between organs of the lateral line and inner ear, as well as mechanosensory cells of invertebrates. The master control gene hypothesis is used to describe evolution of the inner ear, but the authors believe that a single gene is insufficient to produce a state of differentiation. Evidence suggests that all mechanosensory cells are derived from a single ancestor and that they formed the developmental precursor of the ear. The ear appears to have then evolved by an increase in sensory epithelium along with changes in hair cells and supporting structures. Although the authors discuss the molecular basis of the evolution of hearing, more data are required to complete the evolutionary picture. Humans have two cochleae (left and right), which are mirror-shaped, coiled, fluid-filled, bony tubes. The authors review the anatomy of the cochlea (some exquisite images showing fine detail are presented), including its fine structure using transmission electron microscopy and scanning electron microscopy. The focus of the article is how the aspects of the cochlea affect electrical stimulation, especially the sensory targets. Interest in cochlear anatomy persists for surgical approaches to electrically stimulate the auditory nerve. Studies have shown many anatomical variations of the cochlea, which have important implications for inner ear surgery. Narrowing or coiling of the cochlea can affect electrode array insertion. Imaging techniques, such as computed tomography can be used to assess the coiling pattern, which is useful before cochlear implantation surgery. An important issue is the design of electrode arrays and their adaptation to anatomical variations of the cochlea. Electrode arrays can be arranged in various parts of the cochlea, with associated advantages and disadvantages. Ultimately, electrodes need to be designed in a shape that will minimize trauma during insertion. Additionally, smaller and more flexible arrays need to be designed for the various structural variations of the cochlea and to preserve the patient's residual hearing. Ellen C. Jensen* The Anatomical Record