Looping of the primitive heart tube is one of the earliest and most crucial steps in cardiac morphogenesis. Cardiac looping is dependent on normal left-right development, and defects in left-right development result in both heterotaxia and complex congenital heart disease. Single gene defects result in the wide spectrum of heterotaxy phenotypes, and conversely, different gene defects result in similar heterotaxy phenotypes. Elucidation of the molecular-genetic mechanisms of left-right development will greatly increase our understanding of the etiology of this complex group of congenital heart defects.