The general rule of tort law holds a person liable only for a wrong they themselves committed. Vicarious liability is a deliberate, long-recognised exception to this rule: it holds one person -- typically an employer -- liable for a tort committed by another person -- typically an employee -- even though the employer neither committed the wrongful act nor was personally at fault in causing it.

The doctrine does not make an employer liable for absolutely everything an employee does merely because they happen to be employed. Liability attaches only where two conditions are both satisfied: first, that a genuine employer-employee relationship exists between the two parties, rather than a relationship of independent contractor engaged to produce a specific result without day-to-day control over how the work is done; and second, that the wrongful act was committed by the employee in the course of employment -- meaning either while carrying out an authorised task, or while carrying out an authorised task in an unauthorised or improper manner. An employer generally remains free from vicarious liability for an act committed while the employee had, in a phrase courts often use, gone "on a frolic of his own" -- pursuing a purely personal errand entirely disconnected from their employment, rather than merely performing an authorised task carelessly or badly.

An independent contractor's torts, by contrast, generally do not attach to the person who engaged them, precisely because that person does not control the manner in which the contractor performs the work, only the result to be achieved. Where a genuine employer-employee relationship is absent, the doctrine of vicarious liability has no application at all, whatever harm the wrongdoer's conduct may have caused.

Two further principles refine how the doctrine operates in practice. First, an employee remains personally liable for their own tort even where the employer is also held vicariously liable; the injured party may choose to proceed against either party, or both, though they cannot recover the same loss twice over. Second, an act that is expressly prohibited by the employer does not automatically fall outside the course of employment, provided the prohibited act was merely a mode of carrying out an authorised task rather than an entirely different, unauthorised task in substance. A prohibition that changes only how an authorised task is done does not remove the act from the course of employment; a prohibition that defines the very scope of what the employee is authorised to do at all is treated differently, and an act falling outside that scope is not within the course of employment regardless of how the employee characterises their own intentions.