The Fourth Amendment of the U.S. Constitution provides that "[t]he right of the people to be secure in their persons, homes, papers, and effects, against unreasonable searches and seizures, shall not be violated[.]"

Generally this means that the cops need a warrant to arrest you, or search your house, or take your stuff: searches and seizures without a warrant are generally considered to be unreasonable. However, careful readers of the Amendment will note that, although warrants are mentioned, they're in a separate clause, so they're not absolutely necessary to make a search or seizure "reasonable." The U.S. Government, with the support of the Supreme Court, has exploited this ambiguity to create many exceptions to the warrant requirement, of which one will learn if one attends law school.

For (dramatic) example, consider your car. There are at least four ways the cops can get a squint inside.

  1. Consent

    When you get stopped for some reason, the cops will often ask if you mind if they search your car. (After all, why not, if you have nothing to hide?) Most people permit them to do so, and a surprising number get busted for contraband they've got in there.
    YOU DO NOT HAVE TO CONSENT TO A SEARCH OF YOUR CAR!
    If the cop has to ask, it (probably) means he doesn't have reasonable suspicion or probable cause, so you can tell him (politely!) to get bent.
  2. Terry Frisk

    If a cop stops you and has reasonable suspicion that you are armed and dangerous, he is allowed to make a Terry-frisk (a brief pat-down search for weapons in your outer clothing), and if you were a recent occupant of a vehicle, he may look in the passenger compartment (including the glove box and similar places) for weapons (again, if he has a reasonable suspicion that you might have weapons there). Any evidence or contraband he finds in the course of this search is subject to seizure, and you will see it again when you're in court.
  3. Acevedo Search

    If the cop has probable cause to believe that your car contains contraband, he may enter to retrieve it. Enter, here, is a euphemism for break in, which he is allowed to do. He may search anywhere in your vehicle that might harbor whatever it is he believes is there, so if he has probable cause to believe you have very small contraband, he can look absolutely anywhere. He may also open containers he finds within, if whatever he's looking for might be inside.
  4. Chimel/Belton Search

    If, perchance, you are arrested for something, and you were recently an occupant of a vehicle, the cop can search the passenger compartment -- but not the trunk -- for weapons or evidence.

I hope I can remember all the gory details for my test next week...