First-degree murder is the most serious form of murder, and it is defined as the intentional killing of another person by someone who has acted willfully, deliberately, or with planning. The exact definition of first-degree murder varies by jurisdiction, but most state penal codes require that a prosecutor establish willfulness, deliberation, and premeditation in order to convict a defendant of first-degree murder. In California, for example, any murders that are committed with intent and premeditation are classified as first-degree murder.

In order to classify murders in different degrees, criminal law highlights various elements or aspects to take into consideration. A first-degree murder must have three key aspects: premeditation, deliberation, and malice aforethought. Premeditation requires that the defendant had intent to kill and some willful deliberation to kill, rather than killing on a sudden impulse. Deliberation means that the defendant spent some time to reflect, deliberate, reason, or weigh their decision to kill. Malice aforethought means that the defendant acted with a deliberate intention to unlawfully take away the life of another person.

Because first-degree murder constitutes a very serious crime, a conviction results in very serious punishment. In many states, a conviction for first-degree murder can result in the death penalty or life in prison.