Drones, small unmanned aerial vehicles, have emerged from the realm of science fiction to a multi-billion dollar international industry. Advancements and availability of technology have made the practical application of drones a reality for the commercial sector as well as a toy for the hobbyist. Along with the expansion of drone use has also come the opportunity and availability of drones and related technologies for use by threat actors. This paper first introduces unmanned systems and the underlying principles of operation. Next it describes the emergence of drones as tool for industry, but also a potential instrument of malicious activity. Finally, the paper describes the current state of practice in protection against the drone threat including State activities and recommended elements of a counter-drone program.