Normal blood flow conditions during embryonic development are essential for proper heart formation. Altered blood flow in animal models leads to congenital heart defects that recapitulate those in babies with congenital heart disease, and those in animal models with genetic manipulations. How blood flow alters developmental programs leading to heart defects is not well understood. Our studies show that both early cardiac remodeling in response to altered blood flow and the resulting cardiac defects depend on the degree of blood flow perturbation during embryonic developmental stages. Altered blood flow, therefore, is an important player in the development of congenital heart defects. In this talk we will explore possible consequences of our findings.