Franz Joseph Haydn's contributions to classical music were pivotal, as he is often referred to as the "Father of the Symphony" and the "Father of the String Quartet". His innovative compositions helped to define the structure and form of the symphony and quartet, establishing a foundation for future composers. Haydn's use of sonata-allegro form and incorporation of theme development were groundbreaking. Furthermore, his collaborations with key figures like Mozart and Beethoven enriched the classical music landscape. His legacy endures through his influence on musical form and innovation, marking him as a central figure in the transition from the Baroque era to the Classical period.