The writing of a treatise on Rheumatoid Arthritis involves much labour, not only on account of the mass of literature which has to be traced and classified, but, also, and more especially in my case, from the recent discovery of micro-organisms in this disease, implying, in consequence, the revision of all its pathology, morbid anatomy, and treatment. Such an upheaval cannot be accomplished in a day, and this small work is only intended to be an introduction to further and more perfect investigation. Many parts of it will, probably, in the near future require revision, but it has been my ambition to avoid, as far as possible, making any statement unsupported by facts or by logical deductions therefrom. I have here utilized the discovery, of Dr. Wohlmann and myself, of micro-organisms, which we believe to be specific to the disease, and the latters life history and peculiarities, as worked out and elucidated by Dr. Blaxall. To them both my sincere thanks are due.