Middle and inner ear structure and function of seven Odontoceti; Tursiops truncatus, Delphinus delphis, Grampus griseus, Globicephela sp., Orcinus orca, Phocoena phocoena, Phocoenoides dalli, and four Mysticeti; Balaenoptera physalus, B. acutorostrata, Balaeana glacialis, and B. mysticetus were investigated. Measured parameters of middle ear morphology include ossicular mass, density, morphology, and lever ratios, tympanic membrane area, and calculations of middle ear impedances. The inner ear structures described and measured include cochlear morphology, basilar membrane length, width, and thickness, and spiral laminae morphology. Correlations are made between basilar membrane length, width, thickness and thickness/width. Greenwood's equations [D. D. Greenwood, J. Acoust. Soc. Am. 33, 1344–1356 (1961)] are used to estimate auditory thresholds using basilar membrane length and elasticity. Verification of this procedure's accuracy is presented. Inner ear examinations were made using surface preparations, cross sectioning, and scanning electron microscopy. These investigations are designed to complement previous studies in other marine mammals and bats. Results are discussed in light of currently accepted cetacean auditory theory with special attention given to low-frequency auditory adaptations in Mysticeti, high-frequency hearing in phocoenids, and ossicular chain function.