Ever wonder about what an 18th-century physician or surgeon needed to know about skin diseases? For the very first time, the 18th-century handbook, Doctrina de Morbis Cutaneis (1783 edition), has been translated into English. In an effort to distill confusing information about skin diseases into a simple system for his students, Joseph Jacob Plenck (1738–1807)—unknowingly and without any recognition—set the study of skin disease on a path toward formal specialization. Doctrina was the text that inspired Robert Willan (1757–1812) to make his own system; thus, Doctrina is the foundational text of dermatology. In the 250th anniversary of the first edition of this work (1776), it is our intent both to honor Plenck with a translation of his masterful work and to reveal its secrets to modern readers and scholars.