The subject of medical mycology has gained greatly in interest and there has arisen a great realization of the importance of those fungi whose antibiotic products have become indispensable in the treatment of many maladies of Man and other animals. It has therefore, become apparent that an entirely new work is necessary, not merely as a text book for students but also to assist the many persons now undertaking research in the various fields of study in which the knowledge of fungi is fundamental to their work. In the higher fungi, especially the Basidiomyceteae, many new species and genera have been described. Above all, studies in the anatomy and chemical reactions of these new as well as of the old, long known species have shown the artificiality of the old Friesian system of classification. This has led to radical revisions which beyond doubt indicate far more correctly the true relationships and phylogeny of the members of the members of this class. In this work an attempt has been made to give the user the opportunity to learn the more important characters of almost all the generally recognized orders of fungi, and to have a foundation upon which the more intensive study of any of these groups may be based. So many of these organisms are still known but scantily, so that their true relationships are very doubtful. As examples, the Protomycetales and Eccrinales may be mentioned