Mammalian audition is mediated through the organ of Corti (OC), a rigorously patterned mosaic of mechanosensitive hair cells and associated supporting cells arrayed along the long (coiled) axis of the cochlea. Incoming sounds stimulate hair cells along the spiral based on a frequency-place, or tonotopic, code. Using the temporal bones of human cadavers, Georg von Bekesy first showed that decreasing stimulus frequencies elicited motion at increasingly distal locations along the cochlea. Subsequent studies revealed that sound frequencies are carried in parallel streams throughout the brainstem and cortex, demonstrating that tonotopy is the fundamental organizing principle of the mammalian auditory system. In this review we describe cochlear development, tonotopic features of the mature structure, and finally discuss existing knowledge regarding how positional (tonotopic) identity is specified along the auditory organ.