One commonly misunderstood concept in criminal law is that of double jeopardy. The Fifth Amendment to the United States Constitution states that no person shall "be subject for the same offense to be twice put in jeopardy of life or limb." However, this is narrowly read and there are some exceptions, and it applies only to cases where a final decision has been entered.
First, it only applies to the same sovereign, meaning the same government. So the same state cannot put you on trial for the same conduct more than once. However, you can be put on trial by two different states or by a state and the federal government. So, say you're accused of running a scam from your house in Florida over the internet. The government claims that you convinced a woman in Georgia to send you money. Even if you're acquitted in Florida, Georgia can still try you without violating double jeopardy, because it's a separate state. The federal government can also try you for any federal crimes you may have committed at this point.
Double jeopardy also does not apply when you win an appeal. Appealing is when you ask a higher court to review the procedure or decisions of the court that had your trial. If the appeals court agrees with you, they will send your case back to the trial court with instructions on what they need to change. In this case, the court is allowed to have another trial.
The court is also permitted to retry you if there was a hung jury or if the judge had to declare a mistrial. A hung jury means the jury couldn't come to an agreement. Because there must be a legal decision, the case will be retried with a new jury. A mistrial means something went so wrong that it compromised the entire trial. For example, in some high profile cases the jury is sequestered and not allowed to speak with anyone about the case or access any news or media. If a juror violates this rule, the judge may declare a mistrial. Since the trial wasn't completed, there will generally be a new trial.
This article on Double Jeopardy is a guest post from Michael S. Berg , a San Diego based Certified Specialist in Criminal Law who focuses on criminal cases.