Karō (家老, house elder?) were top-ranking samurai officials and advisors in service to the daimyo of feudal Japan.
In the Edo period, the policy of sankin kōtai (alternate attendance)1 required each daimyo to place a karō in Edo and another in the home han (feudal domain). A karō who was in charge of a castle was called the jōdai karō (城代家老), while the one in Edo was called the Edo karō (江戸家老). A general term for a domain-based karō is kunigarō (国家老).
Some domains referred to this position as bugyō (奉行) or toshiyori (年寄).
An example of events involving a karō comes from one of the most famous of all samurai tales, Kanadehon Chushingura. The final Asano daimyo of the Ako han was Asano Naganori. While he was in Edo, he was sentenced to commit seppuku for the offense of drawing a sword against Kira Yoshinaka in Edo Castle. When the shogunate abolished the Ako han, all the Ako samurai became ronin. Oishi Kuranosuke, the jōdai karō, led 46 other ronin in a vendetta against Kira. As a result of his leadership in the Forty-seven Ronin affair, Oishi went down in history as the most famous of all karō.
The shogunate post of rōjū (Elder) had many similarities to that of karō.