Theoretical chemistry is the use of non-experimental reasoning to explain or predict chemical phenomena. In recent years, it has consisted primarily of quantum chemistry, i. e., the application of quantum mechanics to problems in chemistry. Much of this has in fact fallen under the heading of computational chemistry, i. e., the application of computation to the solution of problems in chemistry. Chemical theorists have also used the power of statistical mechanics to provide a bridge between the microscopic phenomena of the quantum world and the macroscopic bulk properties of systems. An aspect of theoretical chemistry is covered by mathematical chemistry in which chemical graph theory plays an important role.

Theoretical attacks on chemical problems go back to the earliest days, but until the formulation of the Schrödinger equation by the Austrian physicist Erwin Schrödinger, the techniques available were rather crude and speculative. Currently, much more sophisticated theoretical attacks based on quantum mechanics are in vogue.