In recent decades, survival of children with complex congenital heart disease has improved considerably. Specifically, children with a variety of congenital heart defects resulting in 'single ventricle' physiology can now undergo palliative surgery that allows survival beyond the neonatal period, and in many cases into adulthood, despite having a single functional ventricular pumping chamber supplying both the pulmonary and systemic circulation. Our growing understanding of the functionally univentricular heart has resulted in freedom from Fontan failure of >50% at 25 years post-Fontan. Yet there is still a fair amount of knowledge to be gained, specifically as it relates to the pulmonary circulation in this group of patients. Knowledge gaps relate not only to the pulmonary circulation after Fontan operation, but also at each stage of the single ventricle surgical palliation, including the native physiology prior to any intervention. The pulmonary circulation is affected by multiple issues related to the single ventricle, including specific details of the anatomy unique to each patient, any intervention(s) undertaken, and potential complications such as aortopulmonary collaterals, protein losing enteropathy, plastic bronchitis, venovenous collaterals, pulmonary arteriovenous fistulae, ventricular dysfunction, pulmonary venous stenosis, and more. This chapter will review the current knowledge with regard to the pulmonary circulation in the single ventricle patient, primarily after the Fontan operation. Additionally, it is our hope to help the practitioner assess the pulmonary circulation in the single ventricle patient; we will also discuss the evidence behind and approach to treatment strategies in order to optimize the pulmonary circulation in this complex group of patients.