Delivery by drone may be legal within two years. Just don’t expect many pizzas or packages to wing their way through your neighborhood by then. Despite huge interest in unmanned aerial vehicles (UAVs), and considerable hype around the idea of using them to deliver goods, experts say significant challenges still need to be solved for drone delivery to get off the ground.

Google and Amazon are leading the development of delivery drones, while UPS, FedEx, and a host of startups are also researching the technology. Earlier this month, the U.S. Senate Transportation Committee drafted a bill that paves the way for regulation of delivery drones within two years. But the technology may need much more time.

While some critics have raised concerns that drones carrying valuable cargo could be shot down or stolen, those issues are trivial next to questions about reliability, autonomy, and coördination with other aircraft. “You have to assume they’ll fall out of the sky,” says Nicholas Roy, a professor at MIT who worked with Google on its drone delivery effort, called Project Wing. “So how do you make sure these vehicles are reliable enough—both the hardware and the software?”

Drones that hijack or bring down delivery drones. Drones that defend delivery drones against drones that hijack or bring down delivery drones. You can easily see how this quickly gets complicated. To read more, click here.