The paper summarizes the use of current technique in the diagnosis of cardiac arrhythmias. The value of non-invasive 24-hr continuous ECG monitoring and signal-averaged electrocardiography has been discussed. The predictive value of circadian changes in RR intervals (HRV) in 24-hr ECG recording was emphasized in patients after myocardial infarction. HRV below 50 ms leads to a 5-fold increase in the risk of death within 3 months after myocardial infarction. The value of signal-averaged ECG in predicting sudden death ion patients after myocardial infarction is also independent of other techniques. Abnormal signal-averaged ECG was found to be a sensitive indicator of susceptibility to sustained ventricular tachycardia. Electrophysiology testing as one of invasive technique, was discussed with respect to its indications and usefulness in selecting non-pharmacological treatment of cardiac arrhythmias (implantation of antiarrhythmic devices, radiofrequency ablation).