Robert Young, loved by millions of viewers as television's all-knowing dad on "Father Knows Best" and the compassionate "Marcus Welby, M.D.," has died. He was 91.

Young died Tuesday evening at his home in Westlake Village, his physician, Dr. John Horton, said today."He just stopped breathing, basically," Horton said. "It was basically related to age, and he had had heart surgery and his heart was not so good."

After a prolific career in films, where Young appeared in such well-remembered movies as "Sitting Pretty," "Northwest Passage" and "Journey for Margaret," he went on to even greater success in two long-running television shows that were among the most popular of their respective decades.

"Father Knows Best," which Young originated on radio in 1949, was moved to television in 1954 and, after a rocky start in the ratings, finished its run in 1959-60 as No. 6. It was so popular that CBS continued it in primetime reruns for two seasons after the original run ended in 1960.

In contrast to the shows where the comedy came largely from a blundering character, "Father Knows Best" aimed for chuckles more than belly-laughs as Jim and Margaret Anderson thoughtfully soothed the growing pains of their Betty, Bud and Kathy.

Answering latter-day criticism that the show wasn't realistic, Young said that adding a subplot about illness or drugs "would have been like taking a beautiful painting and obliterating it with black paint - and that really would have turned the audience off. We never intended the series to be more than a weekly half-hour of fun and entertainment."

He recalled telling a producer friend, in the process of creating the original radio show, "I'd like to be the father, but not a boob." He said they strove to create "what we thought would be representative of a middle-class American family, if there was such a thing. There probably isn't, but that was what we were looking for."

"Marcus Welby, M.D.," which ran on ABC from 1969 to 1976, got even larger audiences with a similarly thoughtful, compassionate lead character. It was the highest-rated show in the 1970-71 season - the first ABC show to be so rated - and was in the top 15 shows for four seasons, 1969-73.

Young's role as the general practitioner who strove to understand patients' hopes and fears as well as their diseases brought him praise from medical groups.

"He's understanding and dedicated," Young once said of his character. "These are words that for some reason have fallen into disuse. I knew from the start that I had to come back to play this man."

He once remarked, "I enjoy acting. Whenever anyone says `retire' I say, `Retire to what?' "

He was married to his wife Betty for more than 60 years, and they had four daughters.