Most frequent pathology seen in a general paediatric ICU is respiratory failure of varying aetiologies and severity. Respiratory support is often needed, requiring a good knowledge of relevant specificities of respiratory physiology and mechanics in the neonate and growing child. To optimise respiratory support to the patient’s individual condition, respiratory monitoring can be helpful and should be thoughtfully used at bedside to anticipate changes in patient’s condition. This chapter reviews and discusses the lung as a functional unit with focus on physiology, pathophysiology and respiratory system mechanics, knowledge that should allow the PICU physician/nurse to optimise respiratory care and support, independent of an individual’s specific diagnosis/disease that was leading to respiratory insufficiency.