The small aortic annulus is a challenge for cardiac surgeons. In this book we are reporting some studies which had as objective the hemodynamic evaluation of the small aortic mechanical and biological prostheses. The small aortic mechanical prostheses St. Jude Regent Nr.17mm is the main objective of the hemodynamic evaluation, although the aortic mechanical prostheses St Jude Regent Nr.19mm and aortic biological prostheses Carpentier-Edwards Nr.19mm were evaluated too. The outcome of such studies demonstrated an excellent outcome regarding the early and mid-term postoperative clinical and functional outcome. Interestingly the postoperative hemodynamic data in terms of left ventricular mass reduction and transprothesic gradients were similar between the aortic mechanical prostheses St Jude Regent Nr.17mm versus aortic mechanical prostheses St Jude Regent Nr.19mm with annulus enlargement procedures. Also the postoperative hemodynamic data were significantly better in patients undergoing aortic valve replacement with an aortic mechanical prostheses St. Jude Regent Nr.17mm than aortic mechanical prostheses St Jude HP Nr.17mm.