1. This presentation summarizes recent research concerning the maturation of sound transmission through the middle-ear conducting system of various vertebrate species. 2. In this review the structural and functional aspects of middle-ear development are presented, as well as the relationships between these events. 3. Functional changes in middle-ear transmission demonstrated that the efficiency of sound conduction improved dramatically during the period of development. 4. The functional development of the middle ear was then compared with the maturation of auditory sensitivity measured from more central locations in the auditory system. 5. The results indicated that conductive development in some species determined the rate of sensitivity development. 6. These findings add an important dimension to our understanding of the underlying processes of hearing maturation.