This article is the third in a series on the biological basis of child health. It outlines how the cardiovascular system develops during gestation and how congenital heart defects (CHDs) may arise in the process. The article details the pathophysiology and treatment of some of the common CHDs, including patent ductus arteriosus, atrial septal defect, ventricular septal defect and transposition of the great arteries. It explains the possible causes of CHDs and explains how these defects are detected and diagnosed. The article also provides an overview of the initial management of acutely unwell infants and children who present with a CHD. The first two articles in the series looked at cells and genetics, and embryology.