The concept of probable cause, the exclusionary rule, and the fruit of the poisonous tree doctrine all guide police and legal authorities in respecting and following the Constitution when it comes to making arrests and admitting evidence into a case. Let's look at each of these.

Probable cause means that there must be a reasonable suspicion that a crime has been committed (in the case of making an arrest) or that evidence of a crime may be at a particular place (in the case of making a search). This concept is meant to protect people's Fourth Amendment protection against illegal arrests, yet it has been discussed, adjusted, and disputed many times over the years.

The exclusionary rule throws out evidence that was obtained through illegal search and seizure. It can also refer to statements that are obtained against the Fifth Amendment (which protects against self-incrimination) or that are obtained without counsel present. The fruit of the poisonous tree doctrine excludes evidence derived from evidence that violates the exclusionary rule. There are exceptions to both of these that include inevitable discovery and independent sources.

Police officers have to be aware of all three of these rules of thumb every day. They cannot, for instance, make an arrest without being reasonably sure that a crime has indeed been committed and that there is evidence pointing to the person being arrested. They have to make sure that legal warrants are in place before carrying out searches, or else the evidence they collect might be excluded. They also have to be sure to inform the people they are arresting of their Miranda rights and make sure that legal counsel is available to those who wish to have it. Further, they must be sure that the chain of evidence is well documented from beginning to end.