Willem Einthoven was a Dutch medical doctor and physiologist known for inventing the first practical electrocardiograph (ECG or EKG) in 1895. His invention revolutionized the field of cardiology by providing a non-invasive tool for recording the heart’s electrical signals, known as electrocardiograms (ECGs). Einthoven’s groundbreaking work earned him the Nobel Prize in Physiology or Medicine in 1924. His legacy endures in modern cardiac diagnostics, inspiring future generations of medical innovators.