Control of coughing is immature at birth; less than half of newborns cough spontaneously or on direct laryngeal stimulation. It is not known whether timing and distribution of motor outflow is optimized for muscle and lung mechanics. The musculoskeletal system is immature. Inspiratory and expiratory pressures appear to be adequate at birth, but the ranges of pressures and volumes actually used by infants during coughing are not well known. Lung structure is immature at birth. Low lung recoil pressures and low tracheal elastance (easy collapsibility) probably combine to reduce maximal gas velocities in uncompressed intrathoracic airways, and relatively low maximal expiratory pressures and high flow-resistive losses may combine to reduce maximal velocities in compressed regions of intrathoracic airways. Better descriptions of normal and pathologic aspects of structure and function are needed, with attention to control, motor, and lung mechanical functions during coughing and forced expiration in human infants. Application to the developing respiratory system of scaling techniques and models of forced expiration, and advances in understanding of two-phase flow in airways, would be helpful.