One of the issues that criminal defense attorneys frequently litigate in the courts, and argue with prosecuting attorneys, is the existence or lack of probable cause. If a police officer who made an arrest did not have "probable cause" to make that arrest, then all of the actions after that point are the result of an illegal arrest and search, and are therefore subject to suppression by the court. As a practical matter, this means that if we can convince a court that there was no probable cause, our client will usually have their charges dismissed and win their case.
According to the US Supreme Court, probable cause to make an arrest exists when an officer has knowledge of such facts as would lead a reasonable person to believe that a particular individual is committing, has committed, or is about to commit a criminal act. The police officer must be able to articulate the facts and circumstances forming the basis for probable cause.
Given the stakes involved, it obviously is very important that we argue in every criminal case that there was no probable cause to stop/arrest/search our criminal defense clients. I have found that judges will bend over backwards to give the benefit of the doubt to police officers in a motion hearing regarding probable cause. I have even seen judges find that an officer did not have probable cause for the crime he arrested a defendant, but he did for another that he did not articulate. Ultimately a dismissal for probable cause is seen as a victory on a technicality, and courts are loath to grant such victories. There are however some judges and courts that are more willing to hold the government to its burden, and will issue dismissals in close cases.
As criminal defense attorneys, is it our job to argue these motions in every case; and force law enforcement and the prosecution to prove that there was probable cause in each case to make the arrest. Often times in negotiations we spend our time convincing a prosecutor that there is a problem with probable cause in a given case, in order to secure a bargaining advantage for our client. Both sides may be unsure as to the result before a judge, and sometimes a compromise can be made that benefits all parties. See out plea bargaining page for more information on the subject.