Many of these improvements involved the combined application of engineering and biological principles to the traditional medical arts, giving physicians new perspectives on the body's workings and new solutions for its ills. From providing better diagnostic tools and surgical procedures to creating more effective replacements for the body's own tissues, engineering helped the 20th century's doctors successfully address such long-standing problems of human health as heart disease and infectious disease.

All through the century, improvements in imaging techniques wrought by the development of new systems—from x-ray machines to MRI (magnetic resonance imaging) scanners—enabled doctors to diagnose more accurately by providing a more exacting view of the body (see Imaging). One of the century's first such diagnostic devices created not a visual, but an electrical, image. In 1903, when Dutch physiologist Willem Einthoven developed the electrocardiograph, he paved the way for a more intensive scrutiny of the heart, spurring others to find better approaches and technologies for fixing its problems.