For most molecules molecular orbital theory predicts a ground-state electronic configuration that is useful for rationalizing relative bond lengths, magnetic properties, and so forth. However, when electron correlation is a dominant consideration, the ground-state configuration may provide a poor representation of the system. In such cases, invoking configuration interaction provides a means of improving the wave function. The aims of this brief tutorial are to explore the basis of the effect and to cite some results that validate the theory. (Contains 2 figures.)