The study presents the modern concepts on QT dispersion which is regarded as one of the reasons of potentially dangerous cardiac arrhythmias. The measurement of QT interval is based on standard 12-lead electrocardiogram or 24-hours Holter monitoring record. The study describes the rules of QT dispersion measurement and the most common problems connected with it. The changes of QT dispersion in ischaemic heart disease, heart failure, cardiac arrhythmia and prolonged QT interval syndrome were discussed. The study presents also the influence of such drugs as amiodarone, sotalol, beta-adrenolytics on QT dispersion. It was found out that QT dispersion measurement in patients with cardiac diseases seems to be the easy predictor of life-threatening cardiac arrhythmias. The problem is still very controversial and needs further clinical investigations.