The sudden disruption of excessive placental supply with fluids and electrolytes is challenging for neonatal physiology during the period of postnatal adaptation. Different from many other nutrients, the body experiences large changes in daily requirements during the first 7-14 postnatal days, and on the other hand does not tolerate conditions of excess and deficiency very well. Imbalances of fluid and electrolytes are common in neonates, which--in addition--might be further aggravated by NICU treatment procedures. Therefore, fluid and electrolyte management can be one of the most challenging aspects of neonatal care of the premature infant. An understanding of the physiological adaptation process to extrauterine life--and how immaturity effects that transition--is the basis which is needed to understand and manage fluid and electrolyte balance in premature infants. This chapter addresses the physiology of postnatal adaptation and other aspects of fluid and electrolyte management (concerning potassium, sodium and chloride) of the preterm infant.