Under criminal law, a principal is any actor who is primarily responsible for a criminal offense. Such an actor is distinguished from others who may also be subject to criminal liability as accomplices, accessories or conspirators.

The legal principle of vicarious liability applies to hold one person liable for the actions of another when engaged in some form of joint or collective activity.

History.

Before the emergence of states which could bear the high costs of maintaining national policing and impartial court systems, local communities operated self-help systems to keep the peace and to enforce contracts. Until the thirteenth century, one of the institutions that emerged was an involuntary collective responsibility for the actions committed by one of the groups. This was formalized into the community responsibility system (CRS) which was enforced by a fear of loss of community reputation and of retaliation by the injured community if the appropriate compensation was not paid. In some countries where the political system supported it, collective responsibility was gradually phased out in favor of individual responsibility. In Germany and Italy, collective systems were in operation as late as the sixteenth century.

While communities were relatively small and homogeneous, CRS could work well, but as populations increased and merchants began to trade across ever wider territories, the system failed to match the emerging societies' needs for more personal accountability and responsibility. In England, Henry the 1st allowed London to opt out of the CRS and to appoint a sheriff and justices in 1133, and between 1225 and 1232, Henry the 3rd assured the merchants of Ypres that none of them "will be detained in England nor will they be partitions for another's debts".

Nevertheless, the idea of imposing liability on another despite a lack of culpability never really disappeared and courts have developed the principle that an employer can incur liability for the acts and omissions of an employee if committed by the employee in the course of employment and if the employer has the right to control the way in which the employee carries out his or her duties (respondeat superior). The imposition of vicarious liability in these circumstances has been justified on the following grounds:

Exercise of control: If penalties are serious enough, it is assumed that rational employers will take steps to ensure that employees avoid injuring third parties. On the other hand, rational employers may choose to rely on independent contractors for risky operations and processes.

Risk spreading: Many consider it socially preferable to impose the cost of an action on a person connected to it, even if a degree removed, rather than on the person who suffered injury or loss. This principle is also sometimes known as the "deep pocket" justification.

Internalizing the social costs of activities: The employer usually (though not always) passes on the cost of compensating injury or loss to the customers and clients. As a result, the private cost of the product or service will better reflect its social cost.

These justifications may work against one another. For example, insurance will increase the ability to do risk spreading, but will reduce incentives for the exercise of control.