Introduction: The shape of the P wave can yield important information about the substrate of atrial fibrillation (AF). On the routine 12-lead ECG, P waves have a smooth appearance. Computer simulation studies with realistic atrial models have shown highly complex P-wave shapes with details that were linked to structural features of the atria. We assessed the true shape of the P wave in control subjects. Methods: We recorded 184-channel high-resolution ECGs in 6 healthy volunteers and averaged over 300 beats for each. Beats were aligned on their P waves. Alignment was based on a compound signal of all 184 channels to reduce the effect of noise on alignment. In addition, 12-lead ECGs were simulated using a single patient-tailored heart-torso model with detailed atrial anatomy. Results: Averaging reduced the noise level to less than 2 microvolt (uV) peak-to-peak. Signal features of a few uV amplitude and less than 5 milliseconds (ms) duration could be reliably distinguished. Measured P waves had 4 to 5 separate peaks that were reproducible between recordings. Simulated P waves demonstrated similar complexity, which was related to structural discontinuities in the computer model of the atria. Conclusions: The true shape of the P wave is very irregular and reflects the complex anatomy of the atria. High-resolution electrocardiography is necessary to reliably assess P-wave shape.