The number of patients with congenital heart disease surviving to adulthood is increasing, with many requiring ongoing medical attention. Although recommendations are that these patients should be cared for in specialist centres, the clinical state of the acutely unwell patient may preclude transfer prior to the instigation of lifesaving treatment. Although the principles of resuscitation in this patient population differ little from those with acquired heart disease, the acutely unwell adult congenital heart disease patient presents a challenge, with potential pitfalls in examination, assessment/monitoring, and intervention. Keys to avoiding errors include: knowledge of the primary pathophysiology, any interventions that have been undertaken, residual lesions present (static or dynamic), and the normal physiological status for that patient-to determine the precise cause for the acute deterioration and to appreciate the effects (detrimental or otherwise) that any supportive and/or therapeutic interventions might have. Expert advice should be sought at the earliest opportunity.