ng quately understood in a considerable part of the solid-state community for many ears. Perhaps the complication of Wigner's important paper of 1934, which con tained a surprisingly good estimate of the correlation energy of a free-electron gas, prompted wishful thinking in the direction of judging such effects to be smaller than he had estimated. At least one serious error of logic appeared in the literature a paper in 1933 by Brillouin@ in which he undertook to sum the Brillouin-Wigner perturbation series to all orders in the electron-electron interaction in a free- of matrix elements in a system of infinite volume, all terms would vanish, so that there would be no correlation energy I suspect that even Slater, although he had taken brilliant account of correlation effects in such things as his paper on ferro entioned above), did not ad te the importance of correlation energy in general. As late as 1951, Lowdin, (@)who had been interacting with Slater's group at M.I.T., published a calculation of the binding energy of sodium that claimed to get agreement with experiment without including any correlation term. Even those who fully appreciated the importance of correlation energy did not in the early years have a very clear conception of all its implications. Thus, for example, Bardeen, who had found correlation effects to be decisively important in the theory of the work function, and who had always included an important correlation term in his calculations of cohesive energy he time gave serious consideration to the prediction of Hartree-Fock theory that ecause of the exchange effect, the density of states in energy should go to zero at the Fermi energy of a free-electron gas at zero temperature. Most of the rest of us were also briefy confused on this point, although Wigner had indicated that correlation effects would destroy this singular behaviour These last sentences touch on the topic of quasi-particles. As noted above, I had had grave doubts when I wrote my thesis that fine details of the dependence of energy on wavevector in one-electron energy bands could have any real physical significance, because correlation effects must surely be large. In the late 1940s, as many points of contact developed between experiments on semiconductors and the theory of their energy band structures, it became clear to many of us that there must exist exact quantum states of the many-electron system with just the same quantum numbers and topology of energy spectrum as the one-electron states of band theory, provided the energy of excitation above the ground state is less than the band gap, i.e., the energy required to produce an additional electron-hole pai at large separation. I do not know who first pointed this out in the literature, or even if it was explicitly pointed out in these early days, but the fact was certainly known to cognoscenti. Later, of course, Kohn and others developed the concept very precisely. Some of us naturally reflected, too, on the possibility that a similar renormalization of single-particle excitation states might occur in metals. While it was clear that for metals one could not define infinite-life quasi-particle states of finite excitation energy, because there was no energy gap the lifetime width an excitation should become small compared with the energy of the excitation as