Critical aortic stenosis is defined as the presence of severe aortic valve stenosis with systemic perfusion that is dependent on right ventricular output through a patent ductus arteriosus. Critical aortic stenosis is not defined by an absolute valve area or gradient because in patients with ventricular dysfunction (either systolic or diastolic) it may exist with larger valve areas and gradients may be underestimated. The degree of left ventricular hypertrophy in these neonates, though not as severe as in older patients, still exists, and the ventricle may be dilated and poorly contractile. This chapter discusses available interventions for the neonate with critical aortic stenosis: surgical aortic valvotomy vs. balloon aortic valvuloplasty in the cardiac catheterization laboratory. Preparation for care of the critically ill neonate in the cardiac catheterization laboratory is outlined, as well as perioperative and anesthetic management considerations for a neonate with critical aortic stenosis.