Theodore Hannigan is the father of Mike Hannigan, portrayed by Gregory Itzin.
Theodore's wife is Bitsy Hannigan. He and Bitsy dislike Mike's new girlfriend, Phoebe Buffay and try to get him to go out with somebody else. Mike then tells Bitsy and Theodore that he loves Phoebe and Phoebe tells Bitsy and Theodore that she threw up in the closet!
At Mike and Phoebe's engagement party, Monica makes Phoebe look like a fool in front of Theodore and Bitsy so Phoebe shouts at Monica.
Theodore and Bitsy's final appearance was at Mike and Phoebe's wedding, in "The One With Phoebe's Wedding".