A knock and talk is an investigative technique used by the police or other law enforcement officers when the officers suspect criminal activity in a private residence or in commercial space such as an office or warehouse, but don’t have sufficient evidence of the illegal activity to obtain a search warrant.

In a knock and talk, the police or other law enforcement officers will approach the door of the residence or commercial space and seek permission from the owner or person who appears to be in charge to “look around” or search the premises. If the owner or person who appears to be in charge gives consent, the consent will usually be effective or valid.

It often happens that after entering the premises with permission, the police or law enforcement officers see evidence of criminal activity in plain view and upon leaving the premises, the officers seek a search warrant from a judge by submitting a statement under oath (an affidavit) describing (1) what they saw in plain view and (2) other evidence that may have been gathered during the course of their investigation.

Based on this affidavit, the judge may sign a search warrant authorizing the police or law enforcement officers to return to the residence or commercial space, seize the evidence they previously observed, and conduct a thorough search of the premises, as described in the search warrant.

The question of whether the police had the right to enter the premises and look around or search for evidence of criminal activity often arises after a person or persons have been charged with criminal offenses (crimes) and seek to suppress (eliminate) evidence discovered by the police during the knock and talk or during the search for which the police received a warrant based on their observations during the knock and talk.

In Missouri, the 'knock and talk' technique is a legal method used by law enforcement when they suspect illegal activity but lack sufficient evidence for a search warrant. Officers may approach a residence or commercial space and request consent from the occupant to enter and look around. If consent is given, it is generally considered valid. During this process, if officers observe evidence of criminal activity in plain view, they may leave and apply for a search warrant by providing an affidavit to a judge detailing their observations and any additional evidence. If the judge issues the search warrant, officers can then legally seize the evidence and conduct a more thorough search. However, the legality of the initial entry and search can be challenged in court, especially if the occupants are later charged with crimes. Defendants may attempt to suppress evidence obtained during the knock and talk or subsequent search, arguing that the initial entry was not lawful or that the consent was not freely given. The court will then assess the circumstances of the consent and the officers' conduct to determine the admissibility of the evidence.