Leonard Bernstein was an American composer and conductor known for his energetic, expressive style on the podium and for composing beloved works like the musical West Side Story. His career took off overnight in 1943 when, as a 25-year-old assistant conductor with no rehearsal, he had to fill in on a few hours' notice for an ill guest conductor at a nationally broadcast New York Philharmonic concert — and triumphed. He went on to become the first American-born conductor to lead a major American symphony orchestra, worked to make classical music accessible through televised concerts for young audiences, and is remembered as one of the most versatile and influential musicians of the 20th century.