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. Instead, the state considers the circumstances of each case, such as the defendant's intent, level of negligence, or recklessness, to determine the appropriate charge and penalty. Manslaughter in Montana is a felony offense and can result in significant prison time. The specific penalties for manslaughter can vary based on the details of the offense and the presence of any aggravating factors that may enhance the penalties upon conviction. It is important for individuals facing manslaughter charges in Montana to consult with an attorney who can provide legal advice tailored to the specifics of their case.