The crime of assault varies from state to state (and under federal law), but is generally defined as intentionally putting another person in reasonable apprehension or fear of an imminent (immediate) harmful or offensive contact. The crime of assault may be committed even if there is no physical contact, as the criminal physical contact constitutes the separate crime of battery. In many states assaulting a police officer or peace officer who is performing official duties (such as making a lawful arrest) is a more serious offense—sometimes a felony.

In Montana, assault is defined under Montana Code Annotated (MCA) 45-5-201. The state differentiates between simple assault and aggravated assault. Simple assault involves purposely or knowingly causing reasonable apprehension of bodily injury in another, or negligently causing bodily injury with a weapon. Aggravated assault is a more serious offense, which occurs when a person purposely or knowingly causes serious bodily injury to another, or uses a weapon to inflict bodily injury. Assaulting a peace officer or a judicial officer is considered a more severe offense under MCA 45-5-210 and 45-5-211, respectively, and can result in felony charges. The law recognizes the increased risk to officers and typically imposes harsher penalties for assaults against them while they are performing their official duties. It's important to note that in Montana, as in many states, the crime of assault does not require physical contact, which would instead be classified as battery in jurisdictions that distinguish between the two offenses.