A lady-in-waiting is a female personal assistant at a court, royal or feudal, attending on a queen, a princess, or a high-ranking noblewoman. Historically, in Europe a lady-in-waiting was often a noblewoman from a family highly thought of in good society, but was of lower rank than the woman whom she attended. Although she may or may not have received compensation for the service she rendered, she was considered more of a companion than a servant to her mistress.

Lady-in-waiting is often a generic term for women whose relative rank, title and official functions varied, although such distinctions were also often honourary. A royal woman may or may not be free to select her ladies, and even when she has such freedom her choices have historically been constrained by the sovereign, her parents, her husband or the sovereign's ministers as, for example, in the so-called Bedchamber crisis.

The duties of ladies-in-waiting varied from court to court, but functions historically discharged by ladies-in-waiting included proficiency in the etiquette, languages, and dances prevalent at court; secretarial tasks; reading to and writing correspondence on behalf of her mistress; embroidery, painting, horseback riding, music making (vocal and/or instrumental) and participation in other queenly pastimes; wardrobe care; supervision of servants; keeping her mistress abreast of activities and personages at court, and discreetly relaying messages upon command.