John Ritter starred as Paul Hennessy, the father, but died on September 11th, 2003, so both him and his character were only in the show for 31 episodes, as Paul dies with him.
Katey Sagal played the mom, Cate Hennessy
Kaley Cuoco played Bridget Erin Hennessy. Her nickname was Beach.
Amy Davidson played Kerry Michelle Hennessy, the youngest sister, even though Davidson is 6 years older than Cuoco. Kerry's nickname was Care Bear.
Martin Spanjers played Rory Joseph Hennessy, the youngest of the clan.
James Garner played Jim Egan, the grandfather who moves in after Paul dies.
David Spade played C.J. Barnes, Cate's nephew who also moves in after Paul dies.