Having the police knock on your door asking to come in can be scary, but you do have rights. Generally, the police cannot enter your home without a legal search warrant or without probable cause to believe criminal evidence is located in your home. Unreasonable searches and seizures violate your Fourth Amendment rights, but there are exceptions that permit warrantless searches.
The plain view doctrine
If you allow the police to enter your home, and evidence of a crime is in plain sight, the police can seize that evidence, even if they do not have a warrant for doing so. Similarly, there is no reasonable expectation of privacy in things you put out for public view. Evidence in public view may be seized even if the police do not have a warrant for doing so.
Sometimes time is of the essence. If police believe that taking the time to obtain a search warrant will lead to the destruction of evidence, threaten public safety or cause you to flee, they may be able to commit a warrantless search and seizure.
If the police suspect you committed a felony and are in the physical process of pursuing you, they can enter your premises to search for and seize evidence even if they do not have a warrant. Note that this only applies to felonies, not misdemeanors.
Know your Fourth Amendment rights
It is important that you know your Fourth Amendment rights, and part of this is understanding exceptions to the warrant requirement. If you believe your Fourth Amendment rights were violated by the police and the search or seizure do not fall under one of the above exceptions, you will want to take the steps necessary to defend yourself against the criminal charges you face.