The piano music of Paul Cooper (1926--1996) has recently become more recognized through two newly released compact discs, Paul Cooper: The Complete Piano Works by pianist John Hendrickson, and New American Masterworks for Solo Piano by pianist John Perry. Cooper's piano music truly deserves more attention. Apart from its extraordinary beauty and expressiveness, it is immediately accessible and communicative. Among Cooper's piano works, Sinfonia (1989), the largest work written in the late period of his compositional career, can be considered a landmark piece. This paper will examine how Cooper successfully integrates classical values with a contemporary language and how such synthesis makes the music more accessible and challenging. More specifically, this paper will discuss Cooper's use of already existing compositional tools such as cyclic form, traditional formal structures, including sonata and ternary form, and a unifying rhythmic motive, as well as his highly personal way of blending them with unconventional uses of atonality and rhythmic formation. In addition, the paper will discuss how Cooper balances the dichotomy of the general atonal scheme and tonal harmonic references through implied harmonic progression and tonal center. Finally, the paper will also explore the various pianistic aspects of Sinfonia, including its remarkable orchestral effects and challenging virtuosity.