This chapter discusses the principle, methods of recording, and the voltage and time calibration lines on the recording paper, time intervals and voltages of different waves of electrocardiogram. The classification of leads as Standard bipolar limb leads, augmented unipolar limb leads and unipolar chest leads and their placement are presented. This chapter also describes Einthoven\'s triangle, Einthoven\'s law, Axis of limb leads and Hexagonal reference system. The definitions of vector, Instantaneous mean vector and Mean QRS vector are provided. Vectorial analysis, construction of normal ECG from various instantaneous mean vectors, determination of Electrical axis of the heart and recording of Vectorcardiogram are explained. The interpretation of ECG and the various abnormalities of rhythm and conduction, myocardial ischaemia and infarction, Ventricular hypertrophy and abnormalities due to changes in serum electrolytes are discussed.