In order for a person to be convicted of a crime, the prosecuting attorney must prove beyond a reasonable doubt that the defendant is guilty.
At trial, a defendant, along with his or her counsel or by self-representation, is given the chance to present a defense that supports whether or not they committed the crime, and if they did commit a crime, why it happened.
In a criminal case, at the start of the trial the defendant will enter a formal response of guilty, not guilty or nolo contendere (no contest) to a charge. This is known as a plea.
Once the plea is entered, a line of defense is presented. The most common defenses that are presented to the court that support the defendant's plea include:
Self defense is a common defense used by people who are charged with a violent crime, like assault and battery (https://www.avvo.com/legal-guides/ugc/self-defense---assault-crimes). The defendant using this defense will admit to committing an act, such as punching someone, but will try to justify their actions by saying the other person involved in the altercation was threatening them. This makes the defendant's actions unintentional and not premeditated. They were a response to a threat against their own safety.
The insanity defense
This defense is used by defendants who do not have control of their own actions or don't understand that they committed any wrongdoing. At times, people suffering from mental disorders aren't able to choose right from wrong. Hence the insanity defense (https://www.avvo.com/legal-guides/ugc/what-is-mental-illness-and-how-does-it-compare-to-insanity-1) prevents those suffering from mental illness to punished the same as criminals. While a person may be found not guilty by reason of insanity, they may serve a longer term in a mental facility compared to a criminal found guilty of the same crime serving time in jail.
The alibi defense
This line of defense says you have evidence that shows you were in another location at the time of a crime. Using this defense often means you have evidence and an eyewitness that can testify to your whereabouts. With sufficient evidence, the alibi defense is a powerful way to clear guilt.
The entrapment defense
Entrapment happens when a person is induced or persuaded, by either law enforcement or its agents, to commit an illegal act that they had no prior intention in doing so. The law does not allow a conviction in this case. However, if the government provided an opportunity for a crime to happen and a person takes advantage of that situation, this is not considered entrapment since the person is ready and willing to commit a crime. Rarely is entrapment a successful defense. It only usually works if the defendant has little or no previous criminal history.
Statute of limitations defense
Most crimes must be charged within a certain period of time from when the crime occurred, this is called the statute of limitations (https://www.avvo.com/legal-guides/ugc/statute-of-limitations-federal-vs-state-crimes). The length of time varies from state to state and crime to crime. More violent crimes, like murder, often do not have a statute of limitations. But for misdemeanors, the time frame can be as little as a few months to a year for the state to charge you. If the statute of limitations passes without a charge, a would-be defendant is free.