Patient's medical history, clinical examination and resting ECG are the cornerstones for diagnosis and treatment of arrhythmias . The majority of severe heart diseases can be excluded with a careful medical history and a thorough clinical examination. For the identification of rare inherited severe arrhythmias it is essential to record the patient's family history. Resting ECG often provides valuable diagnostic clues concerning the aetiology of the symptom. If the patient has a basically healthy heart, his/her functional capacity is normal, the arrhythmia does not cause haemodynamic problems (syncope) and there is no family history of sudden deaths or severe arrhythmias, the arrhythmia is almost without exception benign. Consider an arrhythmia always dangerous if it causes a serious haemodynamic disturbance (syncope or presyncope) or if it is associated with a severe heart disease. The investigation of such arrhythmias demands cardiological expertise and often extensive examinations. It is necessary to make a distinction between arrhythmias of atrial and ventricular origin because this affects both the prognosis and the choice of treatment options. Consider wide-complex tachycardia always to be ventricular until proven otherwise. Ventricular extrasystoles that cause sensations of palpitation, and ventricular tachycardias of a few beats (< 4) in a patient with a basically healthy heart have benign prognosis and seldom require any treatment. The treatment of prolonged (> 30 s) ventricular arrhythmias often require invasive investigations and possibly also the implantation of a cardioverter defibrillator: refer these patients always for more thorough cardiologic examinations. Except for atrial fibrillation, the treatment of a symptomless arrhythmia is only seldom necessary (even a symptomless atrial fibrillation increases the risk of thromboembolism and is an indication for anticoagulation therapy).