This book presents a broad overview on major aspects associated with Atrial Septal Defects (ASDs), which have now become frequent in children as well as in adults. However, their etiology is still largely unknown. Recent evidences of increasing prevalence of ASD may be related use of color Doppler echocardiography. While this book majorly talks about closure of ASDs, there is a special emphasis on the method of creating atrial defects in the fetus with hypoplastic left heart syndrome. Hopefully, this book will present a detailed account of requisite information to physicians caring for infants, children, adults and elderly with ASD which will aid them in providing the best possible care for their patients.