A textbook for senior and graduate chemistry students of all types, with the goal of bringing them to the point where they can understand much of the current chemical literature in which quantum chemical methods or concepts are used as tools. This edition (first was 1978) includes improved coverage of time-dependent phenomena, term symbols, and molecular rotation and vibration; a new chapter on molecular orbital theory of periodic systems; new exercise sets with solutions; and a new appendix that compiles angular momentum rules from operator algebra.