First-degree murder is the most serious form of murder, and it involves 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 states define it as cases involving premeditation and deliberation, while all other intentional murders are defined as second-degree. 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: willfulness, deliberation, and premeditation. Willfulness requires that the defendant acted with the intent to kill another person, while premeditation requires that the defendant had intent to kill and some willful deliberation to kill, rather than killing on a sudden impulse. Prior planning and deliberation are often closely intertwined, and courts generally look for evidence that the defendant deliberated and subsequently formed the intent to kill prior to the act of killing.

It is important to note that the penalties for first-degree murder are severe and can include the death penalty or life imprisonment without parole in many states.