The purpose of this paper is to provide the reader with a picture of what is usually called the "vitamin B complex" and to explain the nomenclature that has been used to designate individual members of the complex. It is apparent to any one who reads a number of papers on this subject that it is difficult to follow the terminology. Confusion has resulted in the past largely because different terms have been applied to newly discovered factors. In addition, specific terms have been coined before the existence of separate entities has been definitely established. Now, however, through chemical isolation and synthesis, through more extensive biologic researches and through clarification of designations, the veil of confusion is lifting and three of the so-called B vitamins are quite clearly discerned. Brilliant researches have elucidated the molecular structure of vitamin B1 and riboflavin, and the existence of a dietary factor or factors