Understanding the basics of foetal circulation and its transition at birth and the neonatal period (first 28 days of life) is essential in managing neonates, particularly preterm infants of varying gestational age. The neonatal period is the most exciting part of human life as abrupt physiological changes in the cardiovascular and respiratory systems occur during it. The maturation of the central nervous system, renal, gastrointestinal, hepatic and endocrine systems with the post-conceptual age is worth knowing in managing various medical and surgical conditions in the neonatal period and their further follow-up.