He lives the American Dream. He built himself from the modest son of Russian immigrant parents to one of the greatest American composers and conductors. And he turned 70 this week.

Composer and conductor Leonard Bernstein celebrated his seventieth birthday yesterday, marking almost three quarters of a century of musical talent and expression. The artist will be honored at a weekend-long celebration at Tanglewood Music Center in Massachusetts, where Bernstein spent almost all of his adult summers, both as a student and as a teacher.

The gala will include songs composed in Bernstein's honor by Stephen Sondheim and others, a performance of Bernstein's "Mass," and performances of works by Haydn and Tchaikovsky which will be conducted by Bernstein.

Bernstein has shared himself with the world through many musical and artistic endeavors. Always, though, he is teaching. "His (Bernstein's) life has always been about one thing teaching and explaining," said conductor John Mauceri in a "USA Today" article.

When conducting, Bernstein often chooses unusual tempos for pieces, exploring a new facet of the work. This enables his audience to discover and appreciate new aspects of the music. He has also written "Young Peoples Concerts," which were televised lessons about symphonic music.

Bernstein's talent is unique in two ways: he is the first American conductor to have gained international acclaim and respect, and he is able to express that talent by both conducting and composing.

Published works say his first trips to Europe were met with skepticism and disbelief; few thought it possible for an American to understand and meaningfully conduct European music. Each time Bernstein conducted a new orchestra, he was met with grumbles and leering looks at the first rehearsal; respect and admiration at the second. He succeeded in proving to all that his talent was real, and American made.

It is unusual for a musician to be able to achieve such high standards in both aspects of music, and Bernstein has had difficulty juggling the two all his life. In his 1982 book, Findings, Bernstein explains his ability to perform both those tasks, as well as write books, compose for the theater and play the piano.

"It is impossible for me to make an exclusive choice among the various activities of conducting, symphonic composing, writing for the theater, and playing the piano. What seems right for me at any given moment is what I must do, at the expense of pigeonholing or otherwise limiting my services to music . . . For the ends are music itself, not the conventions of the music business; and the means are my private problem."

In Making Music Leonard Bernstein, his sister, Shirley, explains the difference between the composer and the conductor: "As a conductor, Lenny can be very excited by the abstract, formalistic, pure music, but as a composer he is drawn to write music that springs from a literary or emotional idea."

Bernstein has not only made news because of his musical abilities, but because of his often controversial life as well. He has supported and worked for the state of Israel, delivered a speech asking his audience to imagine the President issuing orders for the entire country to disarm, and once held a party at his home to raise money for a radical black political group.

In some of his biographies, Bernstein has been called homosexual. In others he is portrayed as a family man. (Bernstein is a widower with three children.) A new biography, by Steven Ledbetter, will be distributed at this weekend's gala. Bernstein has promised to "tell all" in an autobiography, which will be published after his death.

Bernstein did not only work with symphonic music, however. He also became a part of that great American landmark: Broadway. He composed the scores for three major Broadway musicals including On The Town and West Side Story.

West Side Story, perhaps Bernstein's most successful Broadway score, was a collaboration with longtime friends Jerome Robbins and Arthur Laurents.

Work on the play actually began over four years before it was produced. Robbins thought that a play set in New York but based on Shakespeare's Romeo and Juliet might be interesting. He also believed that the conflict between the Manhattan Monticellis and the Kaplans could explore the racial and religious tension of the time. The play was tentatively name East Side Story. The trio was unable to assemble the right mix of song and story, however, and they put the idea to rest.

Four years later, the situation in New York was different. The men met again, changing the story from a war between families to a war between the Jets, an American street gang and the Sharks, a Puerto Rican gang: This time the play took off, opening on Broadway in 1957.

Bernstein's association with Robbins began many years before West Side Story, though, when he composed the music for Robbin's ballet, Fancy Free. Since Bernstein would like to do more ballet composition, he is currently discussing a work with Martha Graham.

Music is not the only important aspect of Bernstein's life, his family is also one of his main loves.

When she was 15, his oldest daughter Jamie, discussed her father with a reporter, "What I like most about Daddy is that he's very human. He's about the most human being I know."