You're sitting at home watching television one night when there's a knock on the door. You're not expecting anyone, so you pull out your phone and check the doorbell camera feed. There is a squad car sitting in the driveway and two officers standing at your door.
You go over and talk to them through the window. They ask, in a rather forceful manner, if you will open the door and let them inside. You turn around to pause the show you were watching, and you take a moment to think over your options.
Do you have to let them in? If you don't, will they force the door open? Can they arrest you for refusing to allow them entry, perhaps saying you were resisting arrest or obstructing their investigation?
This isn't something you have ever been through before. You don't want to make a mistake, and you recognize that the police are the local authorities. Does that mean you have to do what they say?
Let's start with the basics: Unless they have a warrant, you don't have to let them inside. They can ask, but then it's up to you. That's a Constitutional right. It's laid right out in the 4th Amendment, and the police know it. They have to respect your rights.
There are, of course, some rare exceptions. Police may have probable cause that allows them to enter if they think there's a danger to the public or that you're actively committing a crime at the moment, for instance. But, generally speaking, all they can do without a warrant is to ask your permission and abide by what you say.
Will they go get a warrant if you refuse?
They might. In fact, officers who show up at the door may do so to avoid the hassle and paperwork of getting the warrant, but it doesn't mean they can't. They just have to show cause and get the judge to issue one.
If they do, when they come back again, tell them to show you that warrant. You do need to allow them into the house if they have it, or they may have the right to force their way in.
What if they search without a warrant?
Say they don't have a warrant, you tell them they can't come in, and one of the officers opens the door anyway. They come inside against your wishes, find illegal drugs, and arrest you. Now what?
That search was likely illegal, unless they can show probable cause that justifies the entry and the warrantless search. If they can't, the court may throw out evidence obtained during the search -- which includes those drugs they found. Without them, they're likely not going to convict you.
After an arrest
Now you know a bit more about what you should do in a case like this, but you can also see how people make mistakes or allow police to violate their rights. Be sure you know all of the defense options you have after an arrest.