Valentin Roeser’s Essai d’instruction à l’usage de ceux qui composent pour la clarinette et le cor is the earliest treatise on instrumentation and the first theoretical study of the clarinet. Comparisons are drawn with other eighteenth-century instructional materials, e.g. Francoeur’s Diapason général, La Borde’s Essai sur la musique, and Vanderhagen’s Méthode nouvelle et raisonnée pour la clarinette. A history of the chalumeau and two-keyed clarinet is presented, along with a biographical sketch of Roeser and an English translation of the first section of the Essai. Appendices include a checklist of Roeser’s writings and a list of eighteenth-century music using the chalumeau.