Soon after its detection, radio emission from Jupiter was quickly identified as a product of its planetary-scale magnetic field. Subsequent spacecraft investigations have revealed that many of the planets—and even some moons—either currently have or have had in the past a planetaryscale magnetic field. Generated by dynamo processes within the planet, planetary-scale magnetic fields provide a means of constraining the properties of a planet’s interior through remote sensing, and it may even be possible to measure the magnetic fields of extrasolar planets. If so, they will offer one of the few means available of understanding the potential diversity of planetary interiors. In the case of our own planet, the presence of Earth’s magnetic field has long been suspected to be partially responsible for its habitability. Thus, knowledge of the magnetic field of an extrasolar planet may be a valuable component to assess its habitability, or to understand an absence of life on an otherwise potentially habitable planet. This report summarizes the investigations and conclusions from a William M. Keck Institute for Space Studies on planetary magnetic fields.