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The complex physiology of breathing in the newborn has important developmental and maturational aspects. Early in fetal life the respiratory system and controllers respond to intrauterine stimuli (PaCO2PaCO2, PaO2PaO2, and lung inflation). The complex network of control of breathing can be summarized into three basic components: (1) controllers, (2) effectors, and (3) sensors. At birth a multitude of inputs are responsible for initiation of rhythmic ventilation, and adaptation of the pulmonary circulation to extrauterine life. The normal physiology of breathing in the newborn is affected by posture, sleep state, and gestational age. Pathophysiological aspects of breathing in the newborn can be divided into: (1) fetal − pathologies that occur during fetal life and ultimately result in abnormal lung development, (2) immediate postnatal − events that occur primarily as a result of poor or abnormal transition to extrauterine life, and (3) neonatal − events occurring in the neonatal period that lead to intrinsic lung disease or deranged control of breathing such as apnea. This article emphasizes the general importance of understanding the influence of age, posture, sleep state, and maturation on breathing in the newborn, and provides a general overview of these. Paramount to the interpretation of normative and study data is the understanding of the influence of these factors.