The embryonic heart functions as a pump without one-way valves. To accomplish this, a long, slowly conducting myocardial structure, the outflow tract, functions as a sphincter at the arterial pole of the heart. During subsequent development tissue remodeling in the outflow tract and immigrating cells of the neural crest are responsible for connecting the right ventricle with the pulmonary trunk and the left ventricle with the aorta, that is, for the developmental formation of the ventriculoarterial junction. Most congenital malformations of the ventriculoarterial junction stein from disturbances that result in developmental arrest or in abnormal pattern formation ('real' teratology). Abnormal pattern formation can in turn originate from problems with laterality or from aberrant or incomplete formation of structural elements. Genetically modified animals with well- defined gene deficiencies are beginning to provide insight in the signal- transduction pathways and structural elements that are responsible for normal development.