A not guilty plea is one of three possible answers a defendant can provide to a criminal charge. When accused of a criminal charge, such as robbery, the court will ask the defendant how he or she pleads. The defendant can plead guilty, not guilty, or no contest, also called nolo contendre. The plea allows the court to decide how to proceed with the trial. If a defendant doesn't enter a plea at all, whether it is because he or she doesn't want to respond to the charges or because he or she failed to appear, the court will enter a not-guilty plea on the defendant's behalf. While procedure can vary among jurisdictions, most often, a plea of not guilty will be followed by efforts to arrange a jury trial, including setting a court date.

A defendant who enters a not guilty plea is refuting the criminal charges that have been brought against him or her. There are many reasons a defendant may choose to enter a not guilty plea; legal strategy often influences the plea that a defendant may make. An innocent defendant may choose to make this plea in order to have the opportunity to have his or her case tried before a jury, as a guilty plea will not go to trial. A guilty defendant may enter a not guilty plea as well, sometimes for the reason that he or she protests the type of charge being brought.

For example, the general crime of murder has several different types of charges, such as first degree murder, second degree murder, and manslaughter. The differences between these charges have to do with the mental state of the criminal and circumstances around the crime. In addition, the type of charge carries with it different general sentences. For example, first degree murder, generally has a longer and more sever sentence than manslaughter. So, a defendant who may have unintentionally killed someone may plead not guilty to a first degree murder charge because he or she did not have the intent to kill.

Defendants may also choose to enter not guilty pleas because they have a defense to the criminal charge. One type of defense is the insanity defense. If the accused can prove that he or she was insane when the crime was committed, he or she may receive a reduced sentence or avoid sentencing altogether. Jurisdictions vary in how they address insanity defenses.

Obviously, anyone who contests an accusation and wants their day in court will enter a not guilty plea regardless of whether or not they are guilty. Many convicted criminals have pled not guilty, even though evidence showed they committed the crime.

A person can revoke a not guilty plea during the course of a trial. In fact, this is sometimes why a defendant may choose to enter a not guilty plea at first. Changing pleas of guilt and no contest are often looked at with prejudice. Changes to not guilty pleas often occur when a plea bargain is made. The defendant may change a not guilty plea to a guilty plea in exchange for a lesser charge associated with a lighter sentence. The prosecution may offer plea bargains for a variety of reasons. Sometimes, the prosecution will reduce or drop a charge if the defendant, in exchange, provides the government with proof of another person's guilt to another crime.