Global Navigation Satellite System (GNSS) vulnerability is an issue of increasing interest given the importance of GNSS to critical infrastructure worldwide. While the traditional concern has been on denial of service, particularly via jamming, the threat of fallacious position, navigation, or time (termed spoofing) also deserves consideration. Spoofing has been seen around the world and can now be accomplished without significant resources and know-how. It can have deleterious effects due to life safety and automated systems, such as aviation, depending on GNSS. We have been working with the Federal Aviation Administration on developing practical means of detecting spoofing suitable for aviation and other highly-automated systems. This talk will describe our research in this area. While there are no cure-alls, a good combination of these practical detection technologies can make it very challenging and expensive for an attacker to overcome.