The P wave is the first component of the electrocardiogram, and normally precedes the QRS complex and T wave. The period of time from the beginning of the P wave to the beginning of the QRS complex is known as the PR interval.

The P wave represents the depolarisation (electrical activation) of the atria (2 small chambers) of the heart. The P wave is a small, rounded, positive wave in most leads of a 12-lead electrocardiogram, with a maximum height of 0.2 mV and width of 80 ms (both 2 mm on a standard electrocardiogram). The P wave is normally negative in lead aVR, and bifid in lead V1, with an initial positive and final negative component.

While in sinus rhythm (the normal heart rhythm), the size and shape of the P wave is affected only by the dimensions of the atria. Enlargement of the left atrium may cause the P wave to be broader, and sometimes notched, most characteristically when the enlargement is due to mitral stenosis (known as P mitrale). Enlargement of the right atrium, especially in lung disease, may cause a tall, peaked P wave (P pulmonale). In lead V1, enlargement of the right or left atrium may cause an increase in respectively the first and second components of the P wave.

The P wave is interesting in several different arrhythmias (abnormal heart rhythms). In a junctional rhythm (also called a nodal rhythm), the P wave is not visible, as the heart's pacemaker activity is taken over by tissue around the atrioventricular node. In atrial fibrillation, P waves are absent, but smaller multiple "f" or "fibrillation" waves are often present. In atrial flutter, true P waves are absent, but larger flutter waves with both postive and negative components in some leads are apparent. Both atrioventricular reentrant tachycardias (AVRTs) and atrioventricular nodal reentrant tachycardias (AVNRTs) have abnormal, inverted P waves, which occur after the beginning of ventricular depolarisation; they may be buried within the QRS complex and hence invisible on the surface ECG, but sometimes are visible at the end or just after the QRS, more often with an AVRT. Automatic atrial tachycardias have a focus of electrical activity away from the natural pacemaker (the sinoatrial node), and the P wave shape and polarity often indicate the approximate site of that focus within the atria. ventricular tachycardias may demonstrate independent atrial activity (and therefore P waves independent of QRS complexes) in some cases.

Electrolyte disturbances may have effects on the P wave, for example a high blood potassium will lower the P wave voltage, whereas a low potassium will have the converse effect.