This edition of the Smithsonian Physical Tables consists of 901 tables giving data of general interest to scientists and engineers, and of particular interest to those concerned with physics in its broader sense. The increase in size over the Eighth Edition is due largely to new data on the subject of atomicphysics. The tables have been prepared and arranged so as to be convenient and easy to use. The index has been extended. Each set of data given herein has been selected from the best sources available. Whenever possible an expert in each field has been consulted. This has entailed a great deal of correspondence with many scientists, and it is a pleasure to add that, almost without exception, all cooperated generously. When work first started on this edition, Dr. E. U. Condon, then director of the National Bureau of Standards, kindly consented to furnish any assistance that the scientists of that institution were able to give. The extent of this help can be noted from an inspection of the book. Dr. Wallace R. Brode, associate director, National Bureau of Standards, gave valuable advice and constructive criticism as to the arrangement of the tables. D. H. Menzel and Edith Jenssen Tebo, Harvard University, Department of Astronomy, collected and arranged practically all the tables on astronomy. A number of experts prepared and arranged groups of related data, and others either prepared one or two tables or furnished all or part of the data for certain tables. Care has been taken in each case to give the names of those responsible for both the data and the selection of it. A portion of the data was taken from other published sources, always with the-consent and approval of the author and publisher of the tables consulted. Due credit has been given in all instances. Very old references have been omitted. Anyone in need of these should refer to the Eighth Edition.It was our intention to mention in this preface the names of all who took part in the work, but the list proved too long for the space available. We wish, however, to express our appreciation and thanks to all the men and women from various laboratories and institutions who have been so helpful in con-contributing to this Ninth Edition. Finally, we shall be grateful for criticism, the notification of errors, and new data for use in reprints or a new edition.