This is a translation of the second (1858) edition of Berlioz's landmark treatise by Mary Cowden Clarke, daughter of music publisher Vincent Novello. The book was quick to establish itself as a standard work, reflecting Berlioz's keen understanding of the orchestra as both composer and conductor. It is intended as a textbook on the craft of orchestration and to promote better understanding of the essential character of each instrument. Technical details and sonorities are discussed and illustrated with musical examples from composers Berlioz admired, including Gluck and Beethoven, and from his own compositions. This edition includes a section on new instruments, such as the saxophone and concertina, and on the orchestra, and a discussion on the art of conducting. Today the treatise is an important source of information on musical practices of the time and provides us with valuable insight into Berlioz's imaginative and original thinking as a musician.