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.

A defendant generally commits the offense of burglary of a building or habitation (1) by entering a private habitation or any part of a building that was not open to the public, (2) without the consent of the owner, and (3) with the intent to commit a felony, theft, or assault.

A defendant may also commit the offense of burglary of a building or habitation by entering with consent, but remaining hidden with the intent to commit a felony, theft, or assault—or entering with consent and attempting to commit a felony, theft, or assault, or actually committing a felony, theft, or assault.

Some states make a distinction between burglary of a habitation (a place used for overnight accommodations) and burglary of a building that is not a habitation (home). Burglary of a habitation is usually treated as a more serious offense with greater penalties.

In California, burglary is defined under Penal Code Section 459. The state distinguishes between first-degree burglary (residential burglary) and second-degree burglary (commercial burglary). First-degree burglary occurs when someone unlawfully enters an inhabited dwelling with the intent to commit theft or any felony; this is considered a more serious offense and is always charged as a felony, which can lead to a state prison sentence. Second-degree burglary involves entering a commercial establishment or any other type of building that is not a residence, with the same intent, and can be charged as either a misdemeanor or a felony, depending on the circumstances and the defendant's criminal history. California does not require a physical break-in; simply entering through an unlocked door or window with the requisite criminal intent can constitute burglary. Additionally, the state recognizes the concept of 'burglary tools' and penalizes the possession of certain tools with the intent to commit burglary under Penal Code Section 466. The penalties for burglary in California vary based on the degree of the offense, the value of the property taken or damaged, and the defendant's prior criminal record.