Pulmonary arterial hypertension (PAH) in adult patients with congenital heart disease (CHD) is associated with increased morbidity and mortality. The present review aims to discuss the clinical applications of invasive and non-invasive diagnostic modalities and to describe the strengths and weaknesses of each technique. Chest radiograph is an inexpensive investigation providing information on pulmonary arterial and hilar dilatation, pruning of peripheral pulmonary arteries and cardiomegaly. Transthoracic two-dimensional and Doppler echocardiography is the most widely used imaging tool. It provides information on cardiac anatomy and an estimate of haemodynamics and biventricular remodelling and function. In addition, echocardiography is valuable in assessing prognosis and monitoring the efficacy of therapy. Structural and functional changes associated with CHD-PAH, mainly affecting the right ventricle and pulmonary circulation, may represent an ideal target for evaluation with cardiac magnetic resonance. This non-invasive imaging modality has a low biological impact. CT plays an important role for patients with limited echocardiographic windows and those who are unable to undergo MRI (claustrophobia, poor compliance, presence of a pacemaker/implantable cardioverter defibrillator). It is the modality of choice for detailed assessment of pulmonary vessel obstruction or thrombosis. Finally, heart catheterisation remains the gold standard for diagnosing and confirming PAH in patients with CHD and for shunt evaluation. The diagnostic assessment of CHD-PAH requires great expertise and a deep knowledge of both CHD and PAH pathophysiology and should take place in a tertiary centre, where multiple data can be appropriately integrated and applied clinically.