Magnetic fields are a fundamental part of life. They play a role in bird migration and fish navigation, among other phenomena. The earth’s magnetic field enables navigating with a compass. Magnetic fields exist whenever there is electric current; electricity provides a convenient means for creating magnetic fields for other uses, for example, in medicine. Magnetic fields have been used for various medical purposes since at least 2500 BC. Animals and bacteria sense the earth’s magnetic field. The search for how magnetic fields produce biological effects has driven exploration in a multitude of experimental systems, clinical studies, and the clinical use of FDA-approved devices. Various aspects of the effects of applied magnetic fields are understood, while others remain the subject of research. Even so, magnetic fields are used successfully for many medical applications, e.g., magnetic resonance imaging, to speed bone fracture repair, to increase the rate of wound healing, to decrease pain and to treat depression and obsessive-compulsive disorder. Today, there are numerous innovations involving magnetic fields that hold promise in a variety of life science applications.