A "guest conductor" is one who conducts an orchestra regularly, but is not the main conductor. Typically, he or she would be invited by the main conductor to conduct a performance now and then. An "assistant conductor" will often be a young conductor who helps the main conductor and gets the chance to conduct some of the concerts. Leonard Bernstein became famous in 1953 as the assistant conductor of the New York Philharmonic when he led a concert, which was being broadcast nationally on CBS Radio, without having time to prepare for it. He would be the main director of that orchestra from 1958 to 1969.