This reference book contains information about the structure and properties of atomic and molecular particles, as well as some of the nuclear parameters. It includes data which can be of use when studying atomic and molecular processes in the physics of gases, chemistry of gases and gas optics, in plasma physics and plasma chemistry, in physical chemistry and radiation chemistry, in geophysics, astrophysics, solid-state physics and a variety of cross-discipli nary fields of science and technology. Our aim was to collect carefully selected and estimated numerical values for a wide circle of microscopic parameters in a relatively "not thick" book. These values are of constant use in the work of practical investigators. In essence, the book represents a substantially revised and extended edi tion of our reference book published in Russian in 1980. Two main reasons made it necessary to rework the material. On the one hand, a great deal of new high-quality data has appeared in the past few years and furthermore we have enlisted many sources of information previously inaccessible to us. On the other hand, we have tried to insert extensive information on new, rapidly progressing branches of physical research, such as multiply charged ions, Rydberg atoms, van der Waals and excimer molecules, complex ions, etc. All this brings us to the very edge of studies being carried out in the field.