Commercial companies wishing to operate drones in the U.S. face regulatory hurdles—drones up to 55 pounds must fly below 500 feet, only in daylight, and within sight of an operator, who must pass a written exam every two years, according to rules proposed by the FAA.

Would-be drone operators face technical challenges as well, according to Jack Nicas and Greg Bensinger in the Wall Street Journal. They quote Nicholas Roy, a robotics professor at MIT and the former head of Google’s drone-delivery project, as saying delivery drones “are absolutely viable, but there are a lot of technical hurdles that have to be crossed. We are very much in the prototype stage.”

Battery life is the key challenge, followed by inaccurate location data that could, for example, result in your package being delivered to your next-door neighbor’s swimming pool. To prolong battery life, Google experimented with a winged drone that could glide after takeoff, but the design proved hard to control.

Nevertheless, companies are pushing ahead, and Nicas and Bensinger report that Alibaba has used a drone to deliver tea, and a DHL drone delivered medicine to an island in the North Sea. You can read their full article and see a graphic on drone design and delivery methods here (subscription required).