In man, development of the cardiovascular system begins in embryos of about 3 weeks ovulation age and is essentially completed 3 weeks later. Cardiovascular development, therefore, takes place very early and proceeds rapidly. This is a major reason why the study of cardiovascular development is difficult. Events of major importance for the understanding of the processes which lead to cardiac septation and the formation of atrio-ventricular and arterial valves take place so quickly and are so elusive that we still don’t have a thorough understanding of what exactly happens. These uncertainties continue to fuel controversies concerning not only normal cardiovascular development but also the pathogenesis of congenital cardiac defects. Considerable progress, however, has been made in the past few decades [1–9] and the blanks in our knowledge and understanding of heart development are becoming fewer and smaller.