For conductor Michael Tilson Thomas, it's personal. They were friends.
"He could entertain brilliantly and knew how to write music that charmed people. But, he also had profound things to say with his music. Things he deeply felt about."
Thomas met Bernstein when he was in his early 20's.
"He was very friendly and also in testing me the first time we met. He kept asking me about things, which of course I did know."
Respect grew, they shared music and they both started careers at a young age.
Bernstein's work has had a profound effect on American music as well.
"The idea that the work of an American composer could be of lasting impact and importance some way was solidified by the relationship people in the rest of the world had to his music.
There is a three month long tribute to Bernstein at New York's Carnegie Hall beginning next week. With this program by the San Francisco Symphony, concert goers in San Francisco will get a preview.