Attributing old master paintings is one of the most difficult tasks of the art historian. Moreover, the stakes can be high, especially when the painting in question might be that of a famous master. The difference in price between an authentic old master painting and a work 'in the manner of' a well-known painter can add up to several million dollars. The fingerprint of an Old Master discusses issues regarding attribution of Dutch and Flemish paintings of the seventeent-century. Rembrandt's David and Jonathan (Hermitage, St. Petersburg), de-attributed by the Rembrandt Research Project in 1989, provides an unique oppurtunity to compare seventeenth- and twentieth-century conoisseurship. An analysis of the painting based on terms of seventeenth-century art theory shows that the painting must have been successful according to contemporary standards of appreciation, that is, worthy of carrying Rembrandt's name.