Hans Joachim Koellreutter (1915–2009) was a German composer, conductor, and music educator, known for his contributions to contemporary classical music and music education. His genre primarily encompasses avant-garde and modern classical music. Koellreutter studied under notable figures such as Arnold Schoenberg and was deeply influenced by the twelve-tone technique. He is recognized for his innovative compositions that often integrate elements of serialism and tonal structures. Among his notable works are orchestral pieces, chamber music, and educational compositions designed to develop music theory and practice. In addition to his compositions, Koellreutter made a significant impact as an educator. He taught at various institutions, including the University of São Paulo, where he influenced a generation of Brazilian musicians and composers, such as Almeida Prado and Edino Krieger. Koellreutter founded the "Musical Studies Group" in Brazil, promoting contemporary music and fostering a new wave of creativity in the region. His legacy is marked by a commitment to blending traditional techniques with modern concepts, as well as his role in shaping the landscape of 20th-century music in both Europe and South America.