Exercise capacity assessment in patients with congenital heart disease (CHD) is essential for cardiovascular adaptation and is a key parameter in quality of life assessment. In daily practice, the majority of tests are performed at rest, and some even require sedation. Cardiopulmonary exercise testing (CPET) allows the assessment of the patient's haemodynamic status during exercise and provides important information about heart rate, rhythm, ST-segment analysis, arterial pressure, and parameters such as oxygen consumption, oxygen pulse and the ratio of minute ventilation to carbon dioxide produced, which helps to describe in detail the physiology of the patient in a dynamic state. This allows for assessment of aerobic capacity and helps to distinguish the causes of its limitation - cardiovascular, pulmonary or deconditioning. Objectification of exercise capacity provides a basis for better decision-making regarding follow-up plans, exercise recommendations and future interventions. This review will discuss in detail CPET and its implementation in children with CHD.