This paper discusses the Hornbostel-Sachs Classification of Musical Instruments. This classification system was originally designed for musical instruments and books about instruments, and was first published in German in 1914. Hornbostel-Sachs has dominated organological discourse and practice since its creation, and this article analyses the scheme’s context, background, versions and impact. The position of Hornbostel-Sachs in the history and development of instrument classification is explored. This is followed by a detailed analysis of the mechanics of the scheme, including its decimal notation, the influential broad categories of the scheme, its warrant, and its typographical layout. The version history of the scheme is outlined and the relationships between versions is visualised, including its translations, the introduction of the electrophones category, and the MIMO version designed for a digital environment. The reception of Hornbostel-Sachs is analysed, and its usage, criticism and impact are all considered. As well as dominating organological research and practice for over a century, it is shown that Hornbostel-Sachs also had a significant influence on the bibliographic classification of music.