If police officers show up at your home, ask to look around inside, and you tell them no, they need an order from a judge to search against your wishes. On the other hand, if officers want to watch your place from the street, they can, no matter what you think or say.
In public, police can pat you down without a search warrant if they have a reasonable suspicion that you're a criminal.
Now they can also use sophisticated technology to detect things they normally could not see - such as heat coming off your house, or beer on your breath, or a gun under your coat - without your consent, even without your knowledge. Soon, more advanced gadgets may let them peer inside your home to see the heat given off by stoves, lamps, baths and human bodies.
When police officers conduct such high-tech surveillance, have they violated your legal right to privacy? Have they, in effect, searched your home without a warrant? Or have they simply kept a watchful eye out in their effort to protect the public's safety?
The U.S. Supreme Court will soon address those questions in an Oregon case in which police without a search warrant used an infrared camera known as a "thermal imager" to detect heat coming from a home where Danny Lee Kyllo was using powerful lights to grow marijuana.
Police had used the imager to take infrared pictures of Kyllo's house to gather visual evidence - supplementing an informant's tip and electricity billing records - to support a request for a warrant to search the house. When they got the warrant and went inside, they found more than 100 marijuana plants prospering under hot, high-intensity "grow lights."
Kyllo pleaded guilty but appealed the imaging all the way to the nation's highest court. He argued that using the imager constituted a search in itself, requiring a warrant.
Law enforcers, civil libertarians, lawyers and scholars await the court's ruling in Kyllo's case, which is expected by next month. The ruling could set a uniform standard for using the imagers on people's homes and indicate how far police will be allowed to go with other high-tech search and surveillance gear.
"Of course you want to catch the bad guys," said Andy Taslitz, a visiting law professor at Duke University and a former prosecutor. "But if the Supreme Court finds that this is not a search, there's no reason why the police can't drive down the street and scan every house."
As technology improves, police often lag behind criminals in using the latest gadgets. The law, by nature largely reactive, strains to keep current the Constitution's Fourth Amendment protection from unreasonable government seizures of personal property and searches of people's homes, the most private realm of American life.
"Technology is advancing faster than our concepts of privacy," said Taslitz, a Fourth Amendment scholar. "It's going to get more and more complex as technology develops.
"What if it were a case involving explosives or bacterial sabotage? You might feel it's OK to give up some privacy for that. What if police are able to shoot a ray gun at you as you walk down the street to see if you have drugs? Where do you draw the line?"
Police across the country use a wide array of high-tech equipment, including thermal imagers, hidden cameras and audio microphones that pick up conversations in public places, remote microphones that hear conversation through windowpanes, scanners that detect concealed guns, and even flashlights and clipboards with hidden alcohol sensors at drunken- driving stops.
Police say imagers make their job safer and more efficient.
Thermal imagers - cameras that detect infrared radiation invisible to the naked eye - measure heat from objects and show temperature contrasts. (They are not the same as the night-vision equipment that transmitted nighttime images of the war with Iraq; those devices amplify low levels of visible light.)
The blurry thermal images can pinpoint a person fleeing through the woods at night or the waste heat wafting off a roof - whether because of poor insulation, marijuana-growing lamps or a hot drug lab. Police can then use the images to get a warrant from a judge to search inside.
Some police agencies announce when they're trying new surveillance technology in an effort to gauge the public's view of their reasonableness and to allay concerns about secret spying on ordinary, law-abiding citizens.
But that disclosure also may teach attentive criminals what to look out for. For that reason, some police keep their gadgets secret.
"We're afraid that if people knew what we've got, they'd start looking for it," Lt. Ed Sarvis of the Durham, N.C. police said. "The more people know about it, the easier it is for people to defeat it and make it worthless. We don't want to compromise our investigations."
Skeptics say the high-tech surveillance gear goes too far. If cops use it, critics argue, they should get search warrants first to keep it legitimate and protect people's privacy.
"The potential here is for Big Brother action in which we're followed all of the time," said Bob Mosteller, a Duke University law professor. "We think we're protecting ourselves from prying eyes when we go inside and shut the door. Technology threatens the efforts we make to stay private."