Steady advances in eavesdropping technology also have been accompanied by official abuse of it. The government always has billed such technology as an aid to legitimate crime-fighting, and it often has been used for that purpose. But over the years the technology also has been used for political leverage, personal reasons and other inappropriate matters.

Now it is apparent that unmanned aerial vehicles, the drones that once were the province only of the military, will become the latest high-tech instruments at the hands of police and other government agencies.

Congress has instructed the Federal Aviation Administration to clear air space for fleets of drones. Hundreds of companies are developing UAVs for specialized commercial and law enforcement purposes, from mapping to surveillance. Police departments that can't afford helicopters will be able to own and operate relatively inexpensive drones.

Because of the huge potential for privacy invasion posed by drones, federal and state lawmakers should move now to ensure that the devices are used only for appropriate purposes. They should require police to obtain court orders for drone use, backed by sufficient probable cause, just as they would have to obtain approval for search warrants.

The advent of drones is another chapter in the perpetual struggle between liberty and security. Lawmakers should advance both.