Burglary is generally the unlawful entry of a building or structure to commit a felony criminal offense (crime), theft, or assault. The definition of burglary varies from state to state (and in the federal system), and some states have different degrees of severity of the burglary offense—depending on the nature of the unlawful entry (type of building or structure), the intended purpose, and the illegal action taken by the defendant once inside the building or structure. In some states, breaking into an automobile, a coin-operated machine, or an ATM (automated teller machine) with the intent to commit a theft may also constitute the crime of burglary.

In Illinois, burglary is defined under 720 ILCS 5/19-1 as knowingly entering or without authority remaining within a building, house trailer, watercraft, aircraft, motor vehicle, railroad car, or any part thereof, with the intent to commit therein a felony or theft. This does not include motor vehicles, which are covered under a separate statute for burglary (720 ILCS 5/19-2). The severity of the offense can be elevated to residential burglary (720 ILCS 5/19-3), which is a Class 1 felony, if the perpetrator enters the dwelling place of another with the intent to commit a felony or theft. Additionally, Illinois law recognizes possession of burglary tools (720 ILCS 5/19-2) as a Class 4 felony. The specific circumstances of the burglary, such as the type of structure entered and whether the entry was forcible, can affect the charges and potential penalties. It is important to note that the intent to commit a felony or theft must be present at the time of the unlawful entry for the act to be considered burglary in Illinois.