Manslaughter is the unlawful killing or homicide of a human being without the premeditation or malice (malice aforethought) required for the criminal offense of murder. Many states have two types of manslaughter: (1) voluntary manslaughter—the defendant intended to kill the victim, but did so in self-defense, or in the heat of passion and without premeditation, or the defendant was insane and did not understand that the killing was wrong; and (2) involuntary manslaughter—the defendant negligently or recklessly caused the death of another person—such as when a person is intoxicated while operating a motor vehicle and causes an accident that results in the death of another person.

Some states do not have separate criminal offenses for voluntary and involuntary manslaughter—these states may have the offense of manslaughter and identify certain aggravating factors that may enhance the penalties upon a conviction for manslaughter. Manslaughter is a felony offense and may be punished by a lengthy prison term. Manslaughter laws are generally located in a state's statutes—often in the penal or criminal code.

In Montana (MT), manslaughter is defined under Montana Code Annotated (MCA) 45-5-104 as the unlawful killing of a human being without malice aforethought, either express or implied. It is distinguished from murder by the absence of premeditation. Montana law does not explicitly categorize manslaughter into voluntary and involuntary types as some other states do. However, the state does recognize circumstances that can affect the severity of the charge, such as being committed under the influence of extreme mental or emotional stress for which there is reasonable explanation or excuse. Manslaughter in Montana is considered a felony, and the punishment can be severe, including possible imprisonment. The specific circumstances of the killing and the defendant's state of mind at the time can significantly influence the legal outcomes and penalties associated with a manslaughter charge in Montana.