The present volume covers a thrilling two-year period in twentieth-century physics, for during this time matrix mechanics—developed by Werner Heisenberg, Max Born, and Pascual Jordan—and wave mechanics, developed by Erwin Schrödinger, supplanted the earlier quantum theory. In extensive exchanges with the creators of the new approaches, Einstein quickly recognized their great importance and the conceptual peculiarities involved. From the beginning he preferred wave mechan- ics over matrix mechanics. He thought he had found a convincing refutation of the probabilistic interpretation of quantum mechanics in what would today be called a hidden variable theory, but he retracted the paper before publication. Einstein also continued to work on other topics. In early 1925 he had turned to a new mathematical foundation of unified field theory that generalized Arthur S. Eddington’s affine approach on which most of his previous attempts at a unified theory had been based. But he soon abandoned this approach, and in 1927 returned to a different one that he had earlier dismissed: the idea of Theodor Kaluza, further developed by Oskar Klein, that gravity and electromagnetism can be unified by in- troducing a fifth spacetime dimension. Between these two approaches, and inspired by detailed correspondence with the mathematician G. Y. Rainich, Einstein ex- plored features of general relativity in the hope of finding new hints at how the cor- rect unified field theory might look. This correspondence eventually brought about the important Einstein-Grommer paper of 1927, in which they aimed to derive the motion of particles subject to gravitational fields from the gravitational field equa- tions themselves. At the same time, Einstein discussed the interpretation of general relativity and unified field theories with the philosopher Hans Reichenbach. It is here that we find the first statements expressing his decade-long opposition to the idea that general relativity shows that gravity is “geometrized.” In a collaboration with Emil Rupp, Einstein became convinced that Rupp’s ex- periments showed that excited atoms emitted light in a finite time (in waves) rather than instantly (in quanta). However, in subsequent years Rupp’s experiments could not be reproduced. INTRODUCTION TO VOLUME 15