An ostensible agent is a person who appears, from the words or conduct of a principal, to have authority to act on that principal’s behalf, even if no actual authority was granted. In practice, this arises where the principal represents, or allows it to be represented, that someone is their agent, and a third party reasonably relies on that representation when entering into a transaction.The concept is closely linked to “apparent authority” and the doctrine of estoppel. An ostensible agent’s acts can bind the principal as against an innocent third party, particularly in commercial, employment, medical negligence and vicarious liability contexts.In England and Wales and Northern Ireland, the principles are primarily case law–based (for example, on authority and holding-out in agency and partnership law). In Scotland, similar ideas arise through the law of mandate and personal bar. In Ireland, common law mirrors the position in England and Wales.Key issues for practitioners include identifying any representation by the principal, assessing the third party’s reliance and reasonableness, and allocating risk where authority is uncertain or disputed.