A fiduciary duty is the highest standard of care imposed at either equity or law. A fiduciary is expected to be extremely loyal to the person they owe the duty (the "principal"): they must not put their personal interests before the duty, and must not profit from their position as a fiduciary, unless the principal consents. The fiduciary relationship is highlighted by good faith, loyalty and trust, and the word itself originally comes from the Latin fides, meaning faith, and fiducia.

When a fiduciary duty is imposed, equity requires a stricter standard of behaviour than the comparable tortious duty of care at common law. It is said the fiduciary has a duty not to be in a situation where personal interests and fiduciary duty conflict, a duty not to be in a situation where their fiduciary duty conflicts with another fiduciary duty, and a duty not to profit from their fiduciary position without express knowledge and consent. A fiduciary cannot have a conflict of interest. It has been said that fiduciaries must conduct themselves "at a level higher than that trodden by the crowd."