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 Illinois, assault is defined under 720 ILCS 5/12-1 as an act that places another person in reasonable apprehension of receiving a battery without physical contact. Battery, which involves physical contact, is a separate offense under 720 ILCS 5/12-3. Assault is typically classified as a Class C misdemeanor, which can result in fines and up to 30 days in jail. However, if the assault is committed against a police officer, peace officer, or other protected public official or worker while they are performing their official duties, the charge is elevated. Assaulting a police officer or peace officer can be charged as aggravated assault, which is a more serious offense and can be classified as a Class A misdemeanor or a felony, depending on the circumstances, such as the use of a deadly weapon or whether the officer is harmed. This reflects the state's interest in protecting law enforcement and public servants, recognizing the increased risk they face in their official capacities.