The first edition of Gray's Manual was published in 1848. It was to a great extent rewritten and its range extended in 1856, and it was again largely rewritten in 1867. The great advances that have since been made in systematic botany and in the knowledge of our flora have for several years past made another revision desirable, which Dr. Gray before his death was purposing to undertake. The present editors, acting to the best of their ability in his stead, have endeavored throughout to follow his methods and views. The original plan, so long retained by Dr. Gray and so generally approved, has been closely adhered to, the characters and descriptions of the last edition have been left essentially unchanged so far as possible, and in the numerous alterations and additions that have been considered necessary or advisable, his conclusions and principles have governed in every matter of importance, so far as they could be known. The effort especially has been to maintain that high standard of excellence which has always made the Manual an authority among botanists. In the treatment of the genera and species, Gray's Synoptical Flora has been made the basis in the revision of the Gamopetalous Orders, and the manuscript in continuation of that work, so far as prepared, for the Polypetalous Orders which precede _Leguminosæ_ (excepting _Nuphar_, the _Cruciferæ, Caryophyllaceæ, Vitis_, and the small Orders numbered 18, 22, 23, 25--27, and 29). The genus _Salix_ has been rewritten for this edition by M. S. BEBB, Esq., the genus _Carex_ by Prof. L. H. BAILEY, and the Ferns and allied orders by Prof. D. C. EATON. For the rest, all known available sources of information have been made use of, and much willing help has been received from botanists in all parts of our territory. The increasing interest that is taken in the study of the Cellular Cryptogams, and the desire to encourage it, have led to the inclusion again of the Hepaticæ, which were omitted in the last edition. These have been prepared through the kindness of Prof. L. M. UNDERWOOD, though the limits of the volume have necessitated somewhat briefer descriptions than he considered desirable. The three fine plates illustrating the genera of these Orders, which were used in the early editions, are also added, with a supplementary one, as well as an additional one in illustration of the Grasses, thus increasing the number of plates to twenty-five. A Glossary of botanical terms is appended, to meet an expressed need of those who use the Manual alone, and a Synopsis of the Orders in their sequence is given, to contrast more clearly their characters, and to show the general principles which have determined their present arrangement. This should be a useful adjunct to the more artificially arranged Analytical Key.