Left ventricular hypertrabeculation (LVHT) or noncompaction is a myocardial abnormality of unknown aetiology, frequently associated with monogenic disorders, particularly neuromuscular disorders, or with chromosomal defects. LVHT is diagnosed usually by echocardiography by the presence of a bilayered myocardium consisting of a thick, spongy, noncompacted endocardial layer and a thin, compacted, epicardial layer. The pathogenesis of LVHT is unsolved, and the diagnostic criteria, prognosis, and optimal treatment of patients with LVHT are under debate. LVHT is categorized as distinct primary genetic cardiomyopathy by the AHA and as unclassified cardiomyopathy by the ESC. LVHT is usually asymptomatic, but can be complicated by heart failure, thromboembolism, or ventricular arrhythmias, including sudden cardiac death. Mortality of patients with LVHT ranges from 5% to 47%. Anticoagulation is indicated if atrial fibrillation, severe heart failure, previous embolism, or intracardiac thrombus formation are present. In patients with LVHT with late gadolinium enhancement, an implantable cardioverter–defibrillator might be considered if systolic dysfunction, a family history of sudden cardiac death, nonsustained ventricular tachycardia, or previous syncope is additionally present. In this Review, we discuss the current findings on the aetiology and pathophysiology of LVHT, and provide an overview of the diagnosis, available treatment, and prognosis of this cardiomyopathy.