This book originally appeared in Japanese in 1973 in the Iwanami Series of Fundamental Physics supervised by Professor Hideki Yukawa and pub­ lished by Iwanami-Shoten. A revised second edition was published in 1978. The task we set ourselves was to grasp the properties of matter as a whole in a unified scheme and to present a general view of matter incor­ porating the results of modern physics. To achieve this goal we have tried to explore the laws which describe the structure of macroscopic matter, namely, to ask in what kinds of phy­ sical states matter can, in principle, exist and why. Thus, using the meth­ ods of statistical physics and quantum mechanics, we have tried to syste­ matically describe the properties of matter from a unified point of view. Of course, we do not believe that such a standpoint can give an exhaus­ tive description of condensed matter. One of the important viewpoints which obviously is omitted in such a unified approach is the historical one, which follows the development of physics in the course of time.